Huh? Is your argument that it would be bad for Silksong to surprise release next to GTA? Or that indie developers should have planned around the surprise release of Silksong? I really don't understand what argument you're trying to make here.
A lot of devs delayed their launches:
https://www.gamespot.com/articles/silksong-release-date-has-...
Those that didn't or couldn't think it hurt them pretty badly:
https://www.dexerto.com/gaming/hell-is-us-boss-slams-silkson...
In general I think you are probably right. But there are definitely exceptions and this is one of them.
My original point that there is so many things at play, GTA 6 launch date, which may move, together with other things outside of your control, that "planning your release and development around GTA 6" doesn't make sense, unless you're a big company doing a AAA release.
Even if your game is better, you’ll probably make much more money if you don’t time it to launch at the same time as the new GTA
My argument was that other releases can and do impact sales.