Here's the thing though. Throoughout those two decades of different eras, until Project x Zone 2, Leon's never really been in these massive crossovers. like you said, he's more of a loner. And even in stuff like Project x Zone and Project x Zone 2 which didn't have the main villains, they got in as the main faces.
But even the Jacket from the later got in as a DLC costume.
Sure, we can go on and on about how Capcom are advertising him, but that's mainly been for his own series. He's never really made leaps and bounds crossover-wise because like Richter, he's not really considered the face of the franchise. And there doesn't seem to be much precedence for them to all of sudden do so with him in Smash.
We're simply going to have to agree to disagree. Chris and Leon have starred together in one game and one movie. In both of those instances, Leon was the lead, Chris was the backup. Jill hasn't been featured in a game since the 3DS's launch year, while Leon has starred in three movies and two games, including Capcom's expected biggest selling title of 2019.
Again, I
prefer Chris and Jill, but RE is a unique franchise in that they have interchangeable lead characters, and when it came to crossover titles, Chris and Jill benefited from having a more concrete villain, timing and their affiliation with each other. They aren't necessarily
more the face of the franchise or more suited to crossovers by any metric. They just happen to be the beneficiaries of multiple conveniences. If anything, this helps Leon's chances as he would be just as recognizable but
new and immediately more relevant with RE2 Remake's impending release and recent movie appearances.
If any RE protagonist gets into Smash, I want Jill. But I would bet money on Leon.
EDIT: Also, when did Chris get RE6/RE7 costume DLC in MvCi? To my knowledge all he got was the RE1 Classic DLC. Which was
also recycled content from MvC3.