I can understand the idea behind wanting to require the user to specify the value, however, I think keeping the option for the charm developer to provide an opinionated default is important for continuity of the products already available. Maybe a warning provided by the UI during deployment that immutable/constant variables are not being explicitly set would be a good mitigating factor rather than outright requiring the setting of the variables by the operator.
I can understand the idea behind wanting to require the user to specify the value, however, I think keeping the option for the charm developer to provide an opinionated default is important for continuity of the products already available. Maybe a warning provided by the UI during deployment that immutable/constant variables are not being explicitly set would be a good mitigating factor rather than outright requiring the setting of the variables by the operator.