I am not disputing anything you are saying. I am just countering that Matthews + Marner did actually play fairly well together against Tampa... And had they kept that up, then we are probably in the Conference Finals right now.
Now the issue with them not keeping it up is another thing, but it is also a thing that I don't think gets solved by swapping Nylander and Marner. Especially when they are still getting their chances.
If Matthews + Marner were getting completely stifled for opportunities, then maybe it makes sense to switch their lines. But if your players are getting Grade A chance after Grade A chance and simply can't convert, switching up the lines won't do much. Why would you? The lines are working exactly how you want them to work. Players need to do their jobs.
It's an unfortunate recurring theme that I am sure frustrates the coaches and management more than the players. It is basic execution and fundamentals that other teams seem to be able to get more than us. And IDC what coach or what system you are running, you are not having much success if you fumble pucks, can't complete basic passes, make mistakes with limited pressure, or miss Grade A chances time after time. And that is what happened to us. Maurice and Cooper (and the other coaches before them) didn't do anything special other than playing aggressive on the forecheck and clogging up the middle of the ice. Sure that made it hard to just jam pucks in or clean up garbage, but it is not like it prevented us from having more than enough opportunities to score. And then usually at the other end (fortunately not in these playoffs for once), our guys just make a sloppy play or our goalie gives up a weak goal and the other team can just squeak by after that.
I know some people who have worked in NHL front offices and they said that the two most frustrating places to be are either being a mediocre team in limbo so you are not good enough to really make noise but also not bad enough to get picks and players that can set you up for the future, and having a very well-constructed team that can absolutely win but can't get the job done. Because in the end, what do they do? Short of finding some unicorn deal where they get an amazing package which makes the team better by trading one of the better players in the league, their choices are to either stay the course and continue doing their jobs well by setting up the team for success and hope they eventually get the job done, or to make the team worse and finding a new mix of players who can hopefully can get the job done instead. Dubas has chosen the former, but I guess eventually they'll need to go with the latter.