As the developer of Red Stamp Mail I can tell you that there is in fact such a market, albeit small (or perhaps I just suck at marketing).
The problems you describe are exactly what I tried to fix, and users seem very happy to have a simple email application that is free from advertisments, and does not get a "face lift" that makes everything worse every few months.
There's also PawPawMail, which is even simpler and caters to users who need a family member to help them (e.g. manage their contacts list).