Lots of people cite long load times as why they stopped playing GTA5. They are certainly making bank, but they also certainly left money on the table because those players who left could have still been there and spending money.
Them making a lot of money doesn't prove that they couldn't have made more money. Everyone knows better performance means better conversion which means more $$$.
It is definitely non-zero, I think since the launch of game the number of revenue lost to this single bug is in tens, possibly hundreds of millions of dollars. Considering that game made somewhere around 10 billions USD, 1-2% of total revenue lost to loading time seems plausible, given how horrible the problem was.
The players who are still there are used to it so the value is likely near zero. It's the sort of thing that's 100% worth fixing before launch or soon after so you don't lose players in the first place. But the fix loses value every day it goes unfixed because you're already lost those load time-sensitive players. Keep in mind that years after release it still makes Rockstar/Take-Two millions per day (https://www.thegamer.com/gta-5-rockstar-2-5-million-per-day/) so I guess a lot of players just don't care.
For the record: I'm surprised anyone is willing to tolerate that shit.