Not an expert, but have tried a lot of methods for swim improvement, and for my n=1, swimming harder even if the form is sort of breaking down is way better than cutting it short for ‘perfect technique.’ In fact, in my case, I improve way more from swimming really long, really hard, even if I’ve fatigued to the point of really losing reach and pull, than if I don’t. (I rarely go this hard/long, though, and I often swim in excess of 6 hrs per week already).
I’m old enough now to have tried so many approaches, and I’ve found there’s no magic bullet. Invariably for me, training harder = better results. (Obviously, within reasonable limits, you can still overtrain in swimming, just listen to the body fatigue). I’ve tried doing 9-10 short sessions per week vs 3-4 longer ones, mixes of both, all intervals like HIIT style for months with nothing over 200yds, the reverse of that with mostly 500-1000yd steady state swims, focus on OWS, etc. I basically end up swimming the exact same expected speed given the effort and time I’ve put into it, and the type of input doesn’t seem to matter a whole lot at the nonimpressive speeds I’m swimming.
In fact, in my case, if I do NOT push occasionally to the form-breakdown point, I absolutely don’t improve at all. I have to push either the distance or the intensity/rep# of intervals hard enough to really be struggling on the last few. And it’s even better when after I’m toast, I can add another 1000 or more of easier effort swimming on top of it.
Pushing to that point allows you to hold that correct form more comfortably and for a lot longer than you ever could had you just swam within your comfort range with perfect technique. There’s still a time and place for focused all-technique work where you’re not fatigued at all, particularly when fixing specific stroke errors (like those you see on self-video), but even that doesn’t translate to speed in the water until you both ingrain it and then work it hard.