Yea, I'm not sure what they're going to do about players who fill. I played fill the past season, reason being I give my teammates their best role and I'm (usually) competent enough in whatever role I get to be a solid teammate. I think in teambuilder, they actually match you up with better teammates if you're on a champion or role you're unfamiliar with, so there might be something like that?
I think Riot might be subtly pushing players to main in 1-2 roles. The game is getting more and more complex, and this may be one way Riot is addressing the learning curve; by allowing players to master 1-2 roles at a time, instead of forcing players to be minimally competent in all roles to progress.
Playing fill for me started to feel a bit more like a quick-fix to solo-q problems I had, rather than a sustainable strategy to get better at the game. A lot of times I would take Mid or ADC and just play safe, since a lot of players (my opponents) at my elo aren't used to winning unless they snowball, and a lot of Mid and ADC players (my teammates), in my experience, overestimate their ability in the game, or they main some other role and aren't comfortable/competent in Mid/ADC, so they end up doing poorly. The drawback I've started feeling is that I seldom felt like I really took control of a game, I'm just a solid teammate who'd do their job to win a game; I succeeded through flexibility, not mastery, which is a bit difficult to maintain as I climbed because I would have to match an average player in every role I played (especially since I don't play a ton). Like, lets say if I focused on one role, I could be a Gold level ADC (and a Bronze level Mid), but instead I'm a Silver level ADC and Mid.
Personally, I'm just going to start maining Support and Jungle. Especially with Dynamic Queue being like, pseudo ranked-5s, I can find ways to get competent Mid/ADC/Top laners, or at least players I enjoy playing with.