No one is disputing that Marner was the best winger for Matthews for the majority of his career. The term "not being able to fill a void" implies lots of different things. If your benchmark for the Marner void is Matthews' 69 goal season when he was in his mid 20s and ignore the first three years of their respective careers then no one will be able to "fill the void", especially now that Matthews has had recurring injuries since then. Is Nylander at the same level as Marner in terms of producing points with Matthews? No, obviously not, but to "not be able to fill the void" paints a picture like Nylander on Matthews' wing isn't and wasn't also successful, which is simply untrue.
I also don't think the main point was Marner forcing himself onto Matthews' wing, but I agree there is no direct evidence of one person necessarily forcing the coach into anything, but they definitely wanted to play together and that's fine, but that has to be acknowledged that as soon as Keefe got in he tried to make the boys happy by keeping them together almost no matter what. I'd also add that players say a lot of things publicly that just aren't true or at least obfuscate what's actually happening, and if there was any indication of the kind of pull the stars had inside the dressing room I will still never forget Keefe walking back critical comments, so there is plenty of precedent of trying to keep the stars happy even when they weren't playing well.
