There are those who still think C positional value is a thing and that you can't win with out an elite "C". I can guarantee that those same people would have said the same thing about the Blues before they won.
The Step/Brass Rangers were good enough to win, they came close twice and the true deficiencies of that group really lied elsewhere (D, more specifically Staal and Girardi in the top 4.) Even if you subscribe to the C positional value theory, what they have right now 1-2-3 has been as productive (if not more) than most of the teams people would consider contenders - Both Zibanejad (with a rough start) and Strome would be pacing 70+ points in a full year and Chytil would be around 45 over an 82 game season.
Seems kind of silly to sell the farm when it's unnecessary, especially when that farm would go for Eichel who comes with his own list of concerns. I think they'll try as well, but I also don't think they're going to go nuts for him. I get that the entire landscape at the position for the Rangers will shift dramatically in 14 months as both Zibanejad and Strome will be UFA's, but their will be other options available between now and then too. You don't need Eichel to win, every team in the history of the NHL has won with out Jack Eichel at the position and many have won with lesser talents than Eichel down the middle.
Also your time line is a bit off. Brass for Zibanejad was well before the rebuild on the fly. Stepan was really the move that kicked that short lived process off but the situations between this group and that one couldn't be more different. It wasn't so much that Stepan and Brass weren't "elite" enough to win with, but it was about replacing them (or attempting to replace) them with younger players who could contribute more in the coming years. I think most of us would sign up for a replay of the Zibanejad trade right now rather than doing what would be the equivalent of the Rick Nash route with Eichel.