The point isn't how long GM's knew there was going to be an expansion draft. The point was, how long did they know what the exact rules of the draft were going to be? I think it was at least a few months ahead, but after the season starts you can't just trade big salary players willy-nilly, with other teams having their own salary cap issues. And besides, with teams jockeying for playoff position during the regular season, you don't just trade off some big-name talent, even if you can, because of an upcoming expansion draft. What would you think as a fan if the Caps had traded, say, Backstrom, at some point this season for a defenseman and a couple of draft picks because, hey, we need to have spots to protect players in the upcoming expansion draft?
And upon doing some quick research, I found I was wrong -- initial reports of the draft rules were made public in March 2016 for the June 2017 draft. So teams DID have quite a bit of time to evaluate the rules and try to figure out what they wanted to do. But the question still remains: What do you do with your big stars, especially those with NMC? Try to trade them? If so, for whom/what? The established players that other teams would want to give you in return would be those that would be potential problems for THEM vis a vis the expansion draft, so they become your problem. Just trade for prospects and draft picks? The problem is that the best of those prospects are precisely the players that were EXEMPT FROM the expansion draft, so they were considered gold. Any way you cut it, GM's were in a bind. Now did some of them make moves that, in retrospect, turned out to be bad ones? Absolutely. But a lot of those moves were ones that wouldn't reasonably have been expected to pan out for Las Vegas as well as they did.