>>I try to reply short to not keep this thread from exploding.
Having explained my general notion already, I will try to do likewise here to save everybody's time ;). 
Can't make promises, though...you know me anyway :)

>>I feel that there are a lot of emotions around it, but in its core it is just the question: is it good for widelands or bad. 

Given the different thrills that people find playing WL, it will always be more like. "Is it good or bad for _my_way of playing Widelands". Guess, we need to accept this.

>>The game you describe, without military, means that you are playing alone - even in network games, because there is no interaction at all. This is hardly a multiplayer game at all.
True, I said I was provoking with this ;) It served only to make clear that sometimes you have to compromize in order to have a game,that can please more players.
Sorry, if I made you write more than necessary; I know that warfare WL-stye _is_ needed to somehow reap what you have sowed with your economy, and I think WL does a good job at that.

But here is a point that itches me:
I'd like to reverse the principle argumentation of the discussion!
"Tell me what you hate about the tree life cycle." instead of "Show me, where you see any benefit"
Why? 
It don't agree that an existing and working feature, that is liked by many, should have to justify its existance. It should not even matter, why people like it. They simply do. Anyway it's easier to explain annoyances that you have experienced, than to imagine the shortcomings of a world without the feature.

And from this standpoint, we should ask:
"How can those annoyances be fixed?"

This seems to me to be the route this discussion should follow - Not vice versa.
The opponents of the feature do have good points in the argument and I share some of them.
If fixing the annoying points is too cumbersome, or no programmer is willing to work on solutions,it must be dropped. But that is the ultima ratio.

Let me clearly say, that I too have my own points of criticism towards the tree-cycle _as_it_is_. But my stance is to improve the feature, not remove it. It urgently needs tweaking for terrain-affinity, spreading radius and an attenuation of the exponential way trees multiply. (I'm currenty testing values for some of these)

Nobody has up to now said what exactly should be removed, but judging from the arguments  the 8 species and their terrain-affinity is not critical.
By the way - this is what you_do_ notice; that different terrain produces different woods with other tree species.
(Should be optimized, too). 
Among all the properties that those trees have, the only critical one seems that they seed on open terrain and so inevitably slow down the enlargement of a player's territory, and that they re-occupy already cleared space that you want for farms, vinyards and such.

Let's start by the latter:
The main consequence is probably, that you need to keep some evenly placed lumberjacks throughout, as long as you do have some trees near. (natural or planted) to keep your farms, vinyards etc. clear. 
I quite like that requirement; it's reasonable and not asking too much. 
I like the eye-candy part of it as well. A landscape with some few half grown, scattered trees between the houses looks far better, than a completely treeless one.

Now to the forest overgrowth.
I have learnt to hate that, too, as both, a map maker and a player.
Can we all agree, that the woods might be tolerable, if they were not so thick set?
If there was no problem to place a lumberjack or a sentry and connect them inside the forest ?
I would be content if this was the case as a player. These sparse forests would not hinder a player's progress and still the trees would show their natural behavior.
I am currently testing something along that line, in the primitive way that I can. I could do better. if I found some documentation for the statements allowed inside tree conf files.(hint! hint!)

The principle idea, is that of "tree0" that I have failed to explain well enough in my last posting.
So let me try again here. I would like to introduce a 9th tree species into the game. Let's call it "tree0". 
It is an ordinary tree, that does anything that all other trees do. It can be placed by a mapmaker in any number wherever he/she likes (or be not used at all). It will grow, and seed. 
There are only two difference compared to all other trees: They are invisible (their png is totally transparent), and they do not block roads or houses. They do block the space for any other trees though.
In this respect tree0 behaves like the "tiny" version of all trees, or like a dead tree. 
I would give them a terrain affinity of 254 for all terrains, an insane long lifespan, and only a marginal seeding radius of 1 or 2, so that their siblings replace more or less just their fallen anchestors.(tested, looks good)

The task to inlude this species into the game editor, to disallow it to be planted by the rangers, and to make it unrecognizable for the lumberjacks remain however, and are above my head.

So - for the mapmaker.. if you want to keep your harbor space accessible, you place some tree0s there.
If you want to keep some other passage or area clear from tree growth, again place some tree0s there and you are done. 
The other trees will overgrow, but will have no chance to occupy the space that you have reserved with tree0.
You want a map with only very few trees, but still want a terrain useful as farmland?
Scatter enough tree0s, and some others - done.(so far untested)
... and there will still be all other benefits of the tree life cycle going on as usual in the areas that you did not place the tree0s.

The rationales to suggest such a solution is, that even a mapmaker does not need full control over the tree growth on each an every coordinate of the map. He may need to forbid growth in certain areas, but in other places the tree cycle can go on as usual.

Found any flaws in that idea? 
Feasable?
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When I said, that I wished mapmakers had more control over the trees, I had already envisioned the above to be implemened, but I had some further, not very ripened ideas.
The tree0 helps to avoid certain problems, but it isn't what I'd call control over trees. 
The most powerful solution was, if we could make all tree properties from the conf file configurable in the editor - seeing the conf files as the default, but one that can be modified for each map from within the editor.

If this could be achieved, we would of course also have configurable (on a per map basis) properties for tree0, opening up for a more dynamic setup of the open spaces, and (for the first time) scenarios with trees slowly decreasing.(by tree_0 overgrowth or direct modification of there properties) Things like the "birch-pest" could become possible.

I like the idea that map makers can surprise the players, so I don't accept that they should have a clear idea about what to expect.
They need not.