they have to reinvest most of them into the company and it's employees.
In theory, this would be true. But in Mozilla's case, "reinvesting into its employees" means giving the CEO a pay raise in the same year they did huge layoffs. They may be not-for-profit on paper, but the actions from their execs are exactly the same as you'd see from any other for-profit corp. Being not-for-profit is just an excuse for shitty business practices and doesn't change anything in any significant way, imo.
Yeah, they only have a finite number of servers that they can run VMs on, and are pretty consistently at max capacity. That said, there's also limits on individual stream times, so after a certain amount of time you have to reconnect to continue playing (which, if you're playing in a busy time, means re-queueing). So this at least keeps the line moving, in a way.
I haven't tried it in a year or so, but when I played it last, I didn't have very long lines; a minute at most, even at peak hours, on a free (deprioritized) account. Not the end of the world, but definitely an inconvenience.