Yeah, a BM can do other things, but always at the expense of not doing the push. Every time he does something different is one less time he could push. The 'lock does it every time, without fail. Also, the 'lock has quite a few other things he can do other than that even if he takes the 'steep' investment of one of his invocations (agonizing blast is not necessary for repelling blast). Like cast spells, maybe, which the BM will never do. You're comparing the central feature of a subclass that has limited uses to the always on abilities of a 'lock with one of his invocations.
By level 17, you have more than enough superiority dice to have the BM do something every turn. And, frankly, pushing is not that great of an ability. Enemy caster or archer? Pretty much meaningless. Against a wall or someone at the person's back? Meaningless. Speed of 30, hit thrice, and no one with Polearm mastery in the enemy's way? Meaningless. Casting it when you're low level? Generally meaningless.
The BM will always have a variety of options to choose from that are immediately relevant. Pushing is a useful in only niche situations. Its great if used tactically, but that tactic won't be in every round for every battle, nor for every GMing style. Assuming that its meaningful for every attack is unrealistic. Hells, if the enemy has something where approaching them hurts the PCs, Repelling blasts are an active detriment to the party. You can't turn it off.
And, really. I don't know why the BM would even want to push when the vast majority of the time he has better options. Pushing has limited utility in too many situations unless you're setting up a combo with other party members.
And the 'lock will hit far more often that he will miss at 17th. Bounded accuracy ensures this. And that blast isn't much better than a fighter with a bow fails horribly at the exact moment that the fighter runs out of arrows.
I call BS. If you're fighting a hoard of low level monsters with low AC, you'll probably be using AoEs. Even if not, the monsters will be dying fairly quickly to the damage from the blasts, not knockback, and you'll likely have to worry about entire hoards. If you're fighting level appropriate monters, like a high level fiend, you'll be swinging at AC 19; at level 17, you'll have a bonus of +11, assuming maxed casting stat, which is not an assumption that I'm comfortable taking. That's a 40% miss chance, which assumes that 1, maybe 2 will beams will miss. Sure, you might have Foresight, but again, you also might have True Polymorph or Power Word Kill, which are highly useful spells to have.
In the end, assuming that you'll always be hitting with the beam just because high level is frankly unrealistic. Its the type of thing that sounds good on paper, but don't work in execution. People at level 17 are fighting legendary monsters and altering the course of entire planes. If you're fighting mooks, then you're supposed to mow through them like paper. The problem being that there's a lot of paper, and shoving one monster isn't exactly a big deal. And Bounded Accuracy means that those low level mooks
are still a threat to you as well.
