I'm not sure if you grasp the concept of net result. If Matthews's production dips playing with Brown, there is a very good chance Brown's production goes up playing with a top 10 center in the NHL. Net result would be negligible. Plus we're not factoring in players getting better. I see Matthews taking a major step forward this season (health permitted). Tavares turned pretty much every linemate he had in NYI into a stud. There is no reason to believe Matthews won't do the same.
Heres the big difference.
Matthews is not always going to be playing at 100% in form (no player does). Even the very best of the best go thru strechs where their either not fully healthy, fatigued or struggling with their game.
This is why almost every star player also needs another elite talent besides them to create a consistently dangerous line. When that star player takes a dip, his other highly skilled linemate can pick up the slack and create things and put said star in a position to succeed even tho hes not at his best.
Nylander gives this to that line and he's definitely helped Matthews basically be the best even strength goal scorer since entering the league. This isnt to say that Matthews wouldnt have still been great and effective if Nylander wasnt on his wing but Nylander and his high end ability has definitely greatly aided in this.
Nylander is a super fast, zone entry beast, who can both be a playmaker but also someone who can finish. Go back and watch many of Matthews goals last season and many of them start with Nylander coming up with a great play to start the sequence.
Brown is a hard worker, whos responsible defensively and can chip in offensively but in no way is he someone who creates offense on his own or drives posession.
Sure you could put him a line with Matthews but it would be Matthews carrying him all of the time and the minute Matthews drops in form Brown doesnt have the ability/upside to carry that line offensively.
Nylander and Brown are in totally different tiers of offensive players. One guy can create and make things happen on his own while the other cant.
Brown is a guy who on a contending team should play no higher than your 3rd line. I get that he's from Toronto, a late rounder who beat the odds and a extremely hard worker but theres no reason to overrate him.