do you think making engie that threatening/important is bad? i was under the impression that what you're describing can be applied to a good sniper too. pyros if positioned well can disrupt whole ubers, and so on. picks are important regardless of who it is simply because of raw manpower, as well.
the main damage class suddenly becoming not so main-damaging is kind of interesting, to say the least
Let's make this clear, I'm not upset that this weapon makes the engineer threatening or important.
In general, HL is a very slow gamemode. In PL you're artificially limited by the speed of the cart, and on 5cp the speed of your heavy/pyro/engi/spy mean that pushing with momentum often isn't possible, especially on larger maps (CP_Croissant I'm looking at you). Like any major tweak to any weapon, this will make changes in the HL metagame.
The most prevalent change that it will have is that you must kill the engineer before the push, otherwise 70% or more of your damage is just... gone. You're essentially pushing one man down.
The difference between the Engineer in this case and the skilled Sniper and well-positioned Pyro is that there are counters to these classes that are not simply "kill the class." While, of course, killing them is a good idea, pushing through a choke away from the Pyro essentially nullifies him as long as you have a player on your team shooting him during the uber. Pushing through an angle not covered by the Sniper is also an easy way to greatly increase your chances of a successful push.
There isn't another counter to an Engineer with Short Circuit aside from killing him. He can sit by the combo with a dispenser and literally deny any and all projectiles for 2 full minutes, and that's if he's dumb and NOT timing his shots.
There is nothing interesting about a Demoman not being a damage dealer. Pushes in HL succeed because Demomen do damage for their team to clean up. Without a Demoman's damage, other classes are forced to put out more damage, which simply isn't going to be possible with the enormous amount of HP spread across 9 overhealed players. This means more pushes will fail simply because Short Circuit exists. Failed pushes mean more stalemates.
Again, stalemates are basically the farthest thing from interesting.