Are Schiff's concerns valid?

They're valid, but the problem is if you follow them to their logical conclusion - permanent damage of congressional oversight powers and ultimately constitutional crisis - then all you're left with is a massive reset that is too early to speculate on.

Unfortunately, I have very low faith left in Senate republicans - but Trump has personally screwed over so many of them in public, including McConnel and Crux, that is things flip and there is a revolt, it's going to happen exactly the same manner... everyone votes with McConnell. If I was him, I'd flip and keep power in the Senate, because he doesn't need the president for that. But that's just me.