I don't think shortening the draft makes sense because the picks are still trade currency. It is extremely rare that a player is drafted, is rejected by his original team, and signs with a new team and makes the NHL. It certainly happens - Jake Muzzin did it, and Joseph Blandisi managed it here - and there's examples of college players doing it as well.
When I dug into it a few years ago, it looked like around half of the draftees in the 6th and 7th round did not go on to have professional careers of note - they were not top 9 forwards/top 4 D/starting goalies in a major professional league (NHL, AHL, KHL, SHL, SM-Liiga, Swiss League) or 1st pairing D/1st liners in a lesser league (e.g. Germany, Czechia).
Obviously from an NHLPA perspective the draft should be abolished, but I don't think teams hold up players' careers - if they do, it's going to be much more often holding them in the AHL when they have a contract.