I primarily DM. Like 90% of my play time is as DM. I can understand that new material can be challenging for a DM to incorporate into the game, especially given everything else he or she has to do.
But at the same time...5E is pretty streamlined. A subclass is what...a page of text? If even? A feat is a paragraph, maybe two. And other than the races in Volo’s, this is the only other book that players will want to use. It’s not like we’re talking Pathfinder, where there’s a new supplement with new rules every month.
There’s no need to read the whole book. The players can tell him the specific options in which they’re interested, and he can look them over. If he sees concerns, he can explain them. They can have a discussion about it and work it out, like rational people playing a cooperative game as friends.
To me, this thread seems like the internet splitting dowm the middle and digging their collective heels in to prove their side is right. Which doesn’t really help.
I mean, the DM had to expect that they may want to use the new book they had when he decided to TPK them in what sounds like a pretty contrived encounter. Now, we’re only hearing one side, but to me the last thing it sounds like this particular DM needs to hear is that it’s “his game”.
Because it’s not. It’s their game.