Yes many themes are crap but....

Changing theme is not trivial because wodpress core doesn't support it. There is no much point to complain about the quality of themes code when core doesn't provide yet any way to migrate content like widgets and menus between themes,and doesn't provide the ability to costumize them for the new theme before making the switch.

And galleries.... Have anyone ever tried to search a specific gallery? The truth is galleries should be a CPT in core. The rule of thumb is - If 75% of themes do it, then it should be in core, this is how we came to have post formats (ok, I am a little sarcastic).

The idea of moving functionality to plugins doesn't make much sense
1. If the plugin is maintained by someone else you have no control to stop an update from introducing breaking changes to the theme.
2. If the theme author maintains the plugin then he is unlikely to test it against other themes and therefor the compatability is questionable.

The problem here is not with theme authors (even tough 95% of them can't write semi decent php, css or js), it lies with site owners that expect they will be able to make a major design change to their site in 5 minutes.