It’s really simple. Bar owners don’t like customers leaving their venue. Customers leave if it is too loud. Any band should understand that simple business rule.
There’s even more (and I posted about it on that thread.) If someone plays loud, any nuances are not heard, overtones, effects, arrangement details. It all just becomes a sonic mush of air pushed that does not give any breathing room for details.
In some cases musicians play loud as their hearing is bad, due to playing loud music for years without any hearing protection… So it’s a bad circle.
When playing in smaller places, there’s absolutely no need to provide a huge wall of sound. 500W or so concerning PA should be enough. 50W guitar amps are fine. Same with bass amps, 300W should handle it.
I guess metal music is different, but that’s fine. Now, I’ve checked metal band performances now and then but I leave the place after five minutes as there’s no chance to really hear any music the bands are playing, just sonic noise, but that’s my personal opinion. For the rest of the music world, no need to work with huge volumes, that’s ancient.