This only seems applicable if the form factor itself is not a concern for you. If I was to change to a dumbphone, I'd want a long (multiple days) battery life even with moderate usage, physical keys and robustness as a start. Similarly, a good camera or GPS would be features I would expect and hope are excluded from the dumbphones. There's no way to achieve these by taking a smartphone and stripping out software -- there are hardware changes needed.
What you've described seems to me more a 'locked down smartphone' rather than a dumbphone, and the two are not synonyms imho.
Correct. My phone isn't a "dumb phone", but it is certainly "dumbed down". I think that the people partially adopting or experimenting with using dumb phones are doing so to curb habits or escape something. Oftentimes, I think this can be done by simply dumbing down their existing smartphone, or "locking down" as you put it.
What you've described seems to me more a 'locked down smartphone' rather than a dumbphone, and the two are not synonyms imho.