Methinks that there should be a demarcation imposed within the WP owned theme repositories to differentiate between themes that 'have functionality built in, that should be in plugins' and those that 'have functionality from accompanying plugins'. And a visual badge applied as 'branding' to show which camp themes fall into. And a way to view, say, only the latter.

External repositories could use the same badges should they choose to, but I accept that a 'false flag' culture could then arise at the more disreputable end of the market.

If WP doesn't help itself to encourage good coding and standards then the market will dictate the move away. Sort of like the tail wagging the dog?