"The difference is that most proprietary software has already passed most basic quality checks otherwise they couldn't charge for it (or at least continue to charge for it)"

You never had to be the sysadmin, have you really ?

Because if it was he case you clearly would not state something like this. I work in a +10000 computers company (just the users, not the DCs) for more than 15 years and what I can say from all that experience is that the proprietary softwares are not high quality checked.

Examples:
1) microsoft windows (patch Tuesday baby, reboot like you want !)
2) network admin proprietary dashboards like HP, Cisco, Orange, Juniper, Checkpoint, Aruba (sic)
3) AIX. Search for what is that shit and you'll understand, knowing that one server is a 1000000 euros bill a few years ago
4) VMware ... you never know how fucked it is until you use it in production and under load. With a single VM I was able to kill the whole ESX (without knowing it until its death)
5) Any new game coming out crash on most old computers because they do not check correctly a tons of things (video memory space, even some do not check ogl or dx version and just... crash). Yes I have a grudge against Farcry5.

Please also keep in mind that I do not value the FOSS that high. Not much than the average proprietary software. For me both are equals in terms of quality, except that when it's FOSS I can dig inside and fix it.

I agree, there is plenty of great open source software out there, and it still competes with proprietary software, and that's great.