I keep a media gallery of SMF themes from back in the day - I don't have years on them but I could probably figure it out without too much brain power, but there's a definite 'these are mid 2000s themes', and 'these are 2010-2015 themes' vibes about a lot of them. I've often wondered about taking the look from some and applying it to a more modern way of life - for not unreasonable reasons I think themers have tended to avoid going too far off script because too many remember what it used to be like with themes breaking functionality.
Anyway for the curious,
https://smfthemedemos.levertine.org/index.php?media/ - I've got more to post sometime, just haven't gotten around to it. I picked a crap name for it though, perhaps I should have a do-over and rename it to something about forum gallery, and them dump in screenshots of things that aren't just the front pages etc. but also admin panels and stuff like that.
I actually think there's a lot of zest in a lot of them because the last few years has pushed very heavily into a set of design trends (flat, material etc) that I not only don't like, but I find actively user hostile in some respects; we've increasingly reinvented UI paradigms, around 'what is clickable', 'what is actionable', 'what is a button that confirms an action' - there was a much clearer sense of style driven by 'form follows function' that recent design not only fails to reflect but chooses to go out of its way to avoid.
As for the age debate, that certainly explains how come SMF has a pronounced up-tick in 60+ users vs a sample size from XenForo.