I don't want to do this because I think on balance it would hurt performance and confuse users.
Performance because:
a) to do auto-decaching checks it would need to check right back across each theme in the hierarchy
b) it would need to read in theme meta-information to workout hierarchical relationships for each page view
Confuse users:
I see people casually setting theme relationships, but then failing to be able to manage it properly. Frankly we've seen too many cases before where we've added a new level of flexibility, which excites people, and they over-use it. "Too much rope to hang yourself with". For example, a webmaster loves the idea of having 10 websites, so creates 11 themes with inheritance, then forgets about various overrides they have in lower-level themes and can't work out why changes won't show. Or is successful, but is not able to fit everything into their head when a theme-compat breaking upgrade comes along. I know it sounds silly, but this kind of thing happens all the time. Contrast to Wordpress, where people are able to make high quality sites by limiting their ambitions for scope and focusing on design and content (the right way to do it). Obviously we don't want to be Wordpress, but I don't want us to go further down the complexity path.
All themes will inherit from the default theme, so I suggest just editing the default theme and working with that.
Performance because:
a) to do auto-decaching checks it would need to check right back across each theme in the hierarchy
b) it would need to read in theme meta-information to workout hierarchical relationships for each page view
Confuse users:
I see people casually setting theme relationships, but then failing to be able to manage it properly. Frankly we've seen too many cases before where we've added a new level of flexibility, which excites people, and they over-use it. "Too much rope to hang yourself with". For example, a webmaster loves the idea of having 10 websites, so creates 11 themes with inheritance, then forgets about various overrides they have in lower-level themes and can't work out why changes won't show. Or is successful, but is not able to fit everything into their head when a theme-compat breaking upgrade comes along. I know it sounds silly, but this kind of thing happens all the time. Contrast to Wordpress, where people are able to make high quality sites by limiting their ambitions for scope and focusing on design and content (the right way to do it). Obviously we don't want to be Wordpress, but I don't want us to go further down the complexity path.
All themes will inherit from the default theme, so I suggest just editing the default theme and working with that.