Dusty said recently that he wants to use MAT like a 6th man. There's a couple things in there that bother me - the idea that he's proving his worth with great spring stats, the idea that you would give Bryce Harper more than a day off a month - but all in all it's what I'd want to hear. MAT will get enough playing time to see if he's making progress. This is a necessity for the Nats to help make decisions for 2017 and beyond.
The follow-up question is what happens if Werth struggles? Could MAT take over his role full time? If Revere struggles, that would certainly happen. If Bryce struggles enough that MAT could take over for him, the Nats are doomed. But if Werth struggles it is not as clear cut. Ideally yes, MAT would take over. Werth is a liability in the field now, so if he's not hitting his value is reduced to seeing pitches and maybe smart baserunning. MAT on the other hand gives you defensive value even if he's not hitting. It would make sense to pull him.
But how long a leash do you give Werth? It will be tempting to pull him if, say, he gives you a bad April, but that may be premature. It was just 2014 when Jayson hit .292 / .394 / .455, a line we'd kill to have MAT match. Taylor has only hit that level once ever for any extended time in minors or majors (discounting a Puerto Rican Winter league stint), during an extended stay in Harrisburg in 2014. Werth's very good at getting on base, and that's a big skill for this team. (The team's OBP last year without Bryce is a dismal .304) He's also never been a fast starter with the Nats, generally having his best months in the dog days of July and August. Give up on Werth too early and maybe he never gets the reps he needs to get loose. There's also the question of how he'd react. He's shown himself to be a bit... self-important. Could he react to a demotion positively?
This could very well be the question of the early season if Werth struggles. In my mind Werth has earned the benefit of time. However, it all depends on how bad he's hitting.
ZNN is gone and he's not coming back. Most people are ok with it. Ross seemed like a fair replacement for the departing ZNN who dipped a little in his final season in DC. It's almost certainly true that the Nats didn't need to sign a ZNN to a long term deal. However, it's going to be hard for Ross to be ZNN. Over the course of the 2012-2015 window, ZNN was the Nats most valuable pitcher (Strasburg was arguably better but pitched a 130 fewer innings). Not only that he was arguably a Top 10-15 pitcher in the entire league. He's not a Kersahw, Sale, King Felix or yes, Scherzer. However, he was healthy, pitched all the time, and pitched well for four years. He was as valuable as a Cueto, Lester, or Bumgarner.
For the sake of 2016 and beyond there are really two questions. The first is whether Ross can match what ZNN puts up during the same time frame, helping to figure out if the Nats could let ZNN go. That's most likely a yes given their respective ages. At the very least we can agree it's a fair gamble. The second is whether Ross can match what ZNN put up over the last 4 years, helping to figure out if the Nats can expect the same sort of production from him as they got from Jordan. That has to be most likely a no. The hurdle you are asking Ross to clear is to be one of the best 15 pitchers in the majors over the next 4 years. I like Ross but I'm sorry I'm not putting my money on that. ZNN was a special talent when the Nats had him and it's going to be hard to replace that.
With Arroyo down and Giolito being expensive 6 years or so down the road, Roark looks pretty secure in his role as the 5th starter. Is Roark up for it? Roark works if he gets ground balls and keeps the ball in the park. He doesn't walk a lot of players but doesn't strike out a bunch either. His spring has looked ok in those regards - no homers, 9 Ks in 9 IP, but it's spring and the equivalent of one complete game, hardly anything you can hang a hat on. I think the way to look at it is there aren't any warning signs things will be terrible. A bunch of homers (Gott?) or walks (Gio?), or no K's at all (Kelley?). I think a big question for Roark will be not if he can get grounders but if he can get the grounders that he wants. The 3B/SS side of the field should be defended well. The 2B/1B side might be a big issue if Zimm can't move around. Roark can pitch like he wants to and still be beat if the batters can direct their grounders a little bit. If Roark can't do it than next up would almost certainly not be Giolito but Blake Treinen. It could work but I'm not sure anyone would look to that with any sort of confidence.