We disagree.
In the games I play, we encounter different and increasingly diverse opponents, who require more complex tactics, as we go up in level. Flexibility (having more options) is an advantage.
I don't believe we do, actually - not about flexibility, anyway. More options is better than fewer, even if some of those options are marginal or situational, or even, arguably, 'bad' - so long as they're not strictly inferior.
For instance, casters having hundreds of spells is
far better than BMs having 20 maneuvers.
By the same token, those expanding array of choices becoming far more powerful & versatile as you level, is also a tremendous advantage, that the BM doesn't get.
But, if you just meant to disagree specifically, with "That's like a wizard who can only ever know 1st level spells, but, hey, you can upcast them!" - well, yeah, it's not exactly like that, even 1st level spells cover more of a range than maneuvers.

I just meant in a relative sense. Like, the battlemaster needs to better than what it is now, the way the wizard is better than that hypothetical 1st-level-spells-forever wizard.
If you don't agree that hypothetical wizard would be inferior, then, by all means, play a wizard in your next campaign, from 1st to 20th level, without ever learning a spell higher than 1st level, and report back on how it was in no way inferior to the guy casting Wish towards the end of the campaign...