Or you could try watching. Sure there could be some fall off but again, Smith was an established top 6 forward before coming to Vegas. In his first three years playing full seasons of hockey he scored 50, 41, and 50 points. At worst he falls back to a second line winger. But given his chemistry with Marchesseault and Karlsson and how well they play under Gallant's system.
Schmidt is only 3 years into his career and played a much smaller role in Washington. The spike in offensive production is not exactly shocking. Many Caps fans didn't want to see Schmidt go as I remember it and some expressed fondness for him before the Finals got going. But that's just touching on the offense. His defense is easily in the top 4 range of what you want from a guy.
Miller is only 25 and is also only 3 years into his career and, again, got a much more expanded role under Gallant. Will he hit 40 points again? Maybe, maybe not.
But you're buying into Zeke's bull if you can sit there and agree that these guys could regress so hard that they'd be marginal depth guys again. And the fact of the matter is this deal is a quick way to unshelve 3 of there established, veteran, and young defensemen for one guy who Theodore is almost certainly going to be as good or better than, one guy that hasn't lived up to his potential and could likely be paced out by one of Brannstrom, Bischoff, Whitecloud or Hague (probably all four given how they look), and a guy who has two years left in him at best. To say that Miller, Schmidt, and Smith are below the niche of not really essential to Vegas until proved otherwise is A. an easy cop out (as to the let's see them do it again) and B. completely lacking in context and substantive analysis.