While there may well be more player movement, it's no more of an "issue" now than it ever was. Why, you ask?
Because all of this becomes utterly irrelevant as soon as you sit down at a table and ask how that table's DM runs her game; and that is one thing that has never* changed, the best (and IMO misguided) efforts of the RPGA over several decades notwithstanding.
* - exception: you now also need to ask "which edition", not so relevant 30 years ago.
Why?
The only thing that matters is whether you - just you, and maybe the others around your one table - are satisfied with the game and-or rules you are playing right then and there whatever it/they might be. The greater community, the greater game, boards like this: all are secondary to that game at that table, right now.
If my rules and edition are different than yours and both are different from what a third DM uses, etc., so what? Are you happy with what you've got going on at your table? If yes, who cares what everyone else is playing - it just doesn't matter.
Lan-"attempting rules unification in an edition designed to be kitbashed is the fast track to madness"-efan
So what? It means I now have to learn essentially 3 different sets of rules, rather than one. And, if any of those is part of organized play, and there are houserules, it's grounds for complaint, because part of the whole justification for organized play is the ability to drop-in and drop-out and not have to relearn the rules just because you switched tables.
Players now are also more likely to know Jerkface-DM is being Jerkface-DM and not just following the rules, and not making good calls about how he runs the game. Further, 90+% of GM's I've met
do not tell players their house-rules; some of them don't even realize they've made house rules, especially in AD&D1E (where some DM's haven't even found all the supposedly core rules in the PHB... after 20+ years. I find something new in 1E PHB about once a year. Ignoring for the moment that printings 4 and 8 don't have the same content).
And with really fuzzy misinterpretable rules, the problem becomes identifying when your in houserules and or "GM pulling suff out of his derriere" territory.
Hell, in Traveller, many players aren't aware that pre-81 books have different weapon damages and ship building rules than post-81, at least until a guy with post-81 books tries to redo the "standard designs" in order to make a change on a player-owned ship...
Rules matter, as they define the universe as much as the fluff, perhaps more. (Play a D&D module, say, B2, with GURPS or Hero or Warhammer FRP, and each is a very different experience... the rules used by the DM strongly affect how capable characters are, and players should intuit most difficulties before the attempt.)
I don't play the DM; I play the rules. If the DM doesn't, I have a right to know, so I don't waste my time. The DM is the hardware the game runs on, he shouldn't be the game itself.