Here we go again…
TLDR: If it really is that bad and you know you can do better, then get coding. Complaining will get you nowhere and only serve to demotivate the developers.
A little history lesson:
If memory serves, GIMP started out as someone’s student project and was never intended to become anything more than that. In any case, the design had some serious limitations that meant things such as CMYK support and non-destructive editing (including adjustment layers) were very hard or even impossible to add. Despite those limitations the developers somehow managed to make it into something rather competent.
Speaking of developers, while I don’t know what the situation is today, for the longest time the core team was something like three people. Three spare time hobbyist developers working on modernising the core so that the adjustment layers and CMYK brigades could eventually get their toys, while also upgrading the GTK version (that’s not fun), fixing bugs and adding new features relevant to other users. It was a gargantuan task that still isn’t fully finished, but which has finally gotten to the point that version 3.0 can be released. Along the way, various “drive-by” contributors have also added features, but typically because it was something they needed for themselves (they had an itch to scratch). I wouldn’t be surprised if the waves filter was the result of such a need.
And here you are, rather strongly suggesting that your use-case is the only relevant one (as far as I can tell, that would be adding vignettes and not much else) and so the developers should effectively drop whatever they are doing in order to make life easy for you, at the expense of everyone that don’t care about vignettes. But really, other than an entitled demand to add telemetry (which is generally frowned upon in open-source) so that the devs can prove your claims for you, you don’t seem to have much in the way of constructive suggestions on how to turn GIMP into this mean vignette-making machine. I can tell you for a fact that no open-source maintainer will be convinced by that kind of argument and that the only thing it will achieve is making them annoyed with you. You want them to take you seriously? Stop complaining and start contributing. Like with all open-source projects.
Some relevant reading: