Why would you need to replace the instrument?

Because the company made it so it only works with its specific software. Sure maybe you could try and find a way to hack another software in it but that is significantly harder than the stop-gap measures or full-replacement. If you mess up you can end up breaking an extremely expensive tool, and, since funding is extremely limited (talking bare-minimum or even less sometimes), that means you won't risk it.

As opposed to some dude on Lemmy bemoaning that there just can't be solved without source even though I've given actual solutions available now and for little to no material cost?

Yeah well one Lemmy dude actually knows the situation and how things work around a lab and one doesn't seem to understand. It isn't "little to no cost" evidently or most of us sure as shit wouldn't be dealing with stop-gap measures.

You have admitted that you'd still have to rely on someone else's expertise and motivation in the hopes that they'd solve the problem for the lab

There would easily be a team of software engineers who would take on maintaining a lot of the abandonware software we use in a lab since there's a lot of folks who still rely on that software that the company abandoned, including people who know about software more. The key difference you don't understand is that if the source was open it wouldn't be necessary to have an IT enthusiast in every single lab that needs it, you only need 1 or 2 to maintain a repo.

Even then, as I said, they've had decades to figure it out and there exist step-by-step instructions already that are freely available to help them solve the problem or get them almost to the end, assuming, there is some proprietary hardware never mentioned.

First of all, not all abandonware is decades old. Secondly, people are already using the stop-gap solutions that you'd find on the internet, like never connecting the computer to the internet and pray nothing breaks, for example.