I was never a pro... Maybe a -high- level melee samus, and a top level brawl sheik. I can tell you right now, not every pro player goes for the top character or top strategy. You can look at the huge level of difference between the 5 melee gods and see they play what they are comfortable with how they are comfortable with it. They make their play style good.What you say is very true. That does not mean SSB4 will have an organically evolving meta. It will just take a while for the meta to become established because of the large roster plus custom moves. Without balance patches, there's a finite amount of information you can learn from SSB4. I do agree with you that it will take years for the pros to learn everything there is about SSB4.
Character popularity does not really change this much. Once you know all the moves, then you know all the counters to them. If a different move is more popular next month, you just adjust your game play to counter it. You're not learning anything new, just adapting to use a wider bade of knowledge. This hurts the players who know enough to win at the current meta and nothing else. When the popular moves change a new meta evolves and either you know the entire game and just counter the new moves with what you've learnt or you get wrekt for a while because you only learnt up the previous meta and not the entire game's moves.
You could call the above an evolving meta. For the good pros they are not learning anything now but constantly adapting their knowledge the ever changing meta. That is there is not any objectively better characters and movesets. If there is, then this goes out of the window. This is because the pro scene is not about playing who you like, it's about wining, and the pros always use the best.
I agree with you that no one will learn every move and counter anytime soon. I think will mean it'll take a while for a stable meta to occur.
I've played melee for a decade I never even heard of a high level pikachu and now axe is out here nipping at the heels of some of the worlds best!
I understand how now-a-days players feel like patches need to be employed to fix every overbearing aspect (I think this is why everyone is so quick to try and ban... Well everything) but games like melee, and even brawl prove that this just isn't the case.
For years people will be finding solutions to problems different characters and play styles produce. And as one solitIon becomes the new problem people will solve it!
Or I guess they get frustrated and just aim to ban them.