There's speculation about NJ not qualifying Blackwood.
Georgiev was traded for two 3rds and a 5th.
Nedeljkovic for Reimer and a 3rd.
Samsonov wasn't qualified.
Gustavsson was swapped straight up for Talbot.
Vanecek went for a 2nd and a 3rd.
Adin Hill for a 4th.
Vladar for a 3rd.
Goaltenders are fairly cheap these days, especially in that age/dollar range.
A high 2nd isn't bad value at all (~40th overall). Allen is only a stop gap/mentor that may be flipped for a later pick, it's not unusual for a goaltender trade to require one going back the other way.
It takes a much better track record and/or specific circumstances for a goaltender to return a 1st and more. This is about as close to a 1st as you're going to get.
Hart is like dozens of other goaltenders that show high promise and wilt after a couple of years. Sometimes they are in a bad situation and their performance improves with a change in that situation, whether that be the team improving around them or moving to a different team, but it's far from a guarantee.
There are enough possibilities that we don't need to throw a bunch of assets away on a player that is no less of a gamble than someone cheaper to acquire/sign.