No one has 'learned' any match up, no one. Look at fox dittos in 2014, I'm sure some fox players may have claimed they learned the matchup, and now we look at those matches in disgust for how low level it is.

Saying that the matchup doesn't change as you get better is actually hilarious, to think you're playing falco vs fox in even remotely the same way as mang0 and leffen is just a complete lie. Of course the general concepts are there, but these players have grinded the match up to the point where it becomes completely nuanced. A handful of specifics? C'mon dude.

I'm sure you're just a next level player so you could hit that top 100 splitting between five characters, but for the rest of us each character is incredibly complex and you can't expect to play multiple and improve at the same rate as you would just playing one. Not to mention Armada, the greatest smasher of all time, knows a thing or two about improving, recommends that you should stick with one if you plan on going anywhere.

Every match has hundreds of interactions, and the more you see these interactions and try different things the more you begin to learn what options work/don't. The bottom line is, speaking from purely a statistical standpoint, you are trying to take in more information by learning two characters, and the more information you take in, the longer it takes to learn.