The main reason why guys like Phil Davis and Palhares don't get better is because they actually ARE "wrestlers" and "BJJ" fighters.
They are specialists.
Anyone who comes into MMA in their mid to late 20s with a dedicated competition career in a single martial art is going to have difficulty picking the other aspects up with few exceptions to guys who are really athletic or really apt, like Frankie Edgar, Chris Weidman, Jon Fitch.
This is why guys like Cain, Chael, etc are only decent strikers that don't have good submissions but have some of the best wrestling around and beat the hell out of people anyway.
The key is, just like any other sport, to start honing your skills when you are young.
Dan Cormier probably has the best boxing for an older fighter that came in as a specialist, but that's also in a division of the worst athletes, and against the likes of Bigfoot and Josh Barnett. I remember him fighting Devin Cole and looking very unimpressive and just trying to basically takedown and lay in guard for a decision win.
I think the sort of way "bases" come into play is very minor, yet still somewhat important. Obviously Benson's kicking skills and range that he fights from come from his early, prepubescent training in TKD.....even though relatively that's a lesser part of his game.
His biggest MMA skills he developed was his wrestling that he developed in college. His lack of timing is pretty evident and is largely due to not having early training in real striking.
GSP had a karate base but he switched his FOCUS to wresting and submission grappling presumably because he didn't like getting hit. We still see the karate training with the sort of range he keeps and the occasional kicks he throws, but it's secondary because his focus shifted when he was in his early 20s.
And this pretty much goes on and on...