I should follow-up that even if they did implement a model that estimated "true" skill that factors in GP and participation, they wouldn't match you on it. There would be people in Aurodium with the same SR as someone in Carbonite or Kyber...you'd still be in different Leagues because of roster strength. So yes, "skill" is a misleading name in this--but it is getting at what they want: 50% winability. Because whether skill or mods or participation cancel out roster, you're matched against whom you should be matched against.
So while I like the idea of a model estimation procedure that rewards you more for difficult matches, GP alone is not an indicator of a difficult match. Sometimes your higher GP matches are your easier ones, so you might actually be rewarded LESS for beating them under a system like that. And once the model converges, there should in theory be no such thing as more or less difficult matches, as you always have a 50% win rate.
And as a business model, CG is simply not going to reward smaller rosters. One of the major points of the redesign was to prevent people from sandbagging their rosters. Ultimately, roster growth is what they want to encourage, and is the platform for the entire game.