Comment 6 for bug 1439769

The reasons to why old kernels are not intentionally being marked for autoremoval (as of 2008-11-04) is because it was considered too dangerous, and apt has an exception to not tag linux-image-* packages for autoremoval because _we_ had no idea which kernels actually worked for a system.

However, I had thought that this problem had been addressed using last-good-boot for grub after runlevel 2 was reached, here:

https://wiki.ubuntu.com/KernelTeam/removing-old-kernels

But since the wiki hasn't been updated since '08, maybe that is not true anymore.