I don't want to give the creator a click, because I think his content is often clickbaity garbage that has contributed to the very low quality discourse currently surrounding Guilty Gear Strive, a series I care about, immensely.
That said, I think Guest Characters have become an incredibly lame trend in FGs. Obviously, they're great for business, but they've been done so much, that they've become an expectation, instead of a pleasant surprise that should generate any form of genuine hype.
Hell, now that the "Smash Platform Fighter" has become a subgenre supported by larger publishers, the guest character concept loses even more value. If/When a Disney Fighter gets announced, are you going to be surprised when Thumper makes it on the roster at some point?
I appreciate Guilty Gear: Strive and Street Fighter V very highly for avoiding this trend and keeping their characters within their universes/IP stables.
I actually think going forward the fighting games WITHOUT this gimmick are going to be the games that stand out.