I know everyone here loves FOSS, and for good reason, but let's not pretend it doesn't have its own issues. UX and accessibility are two I whine about regularly, but another big one is project abandonment.
I can't tell you how many old forum/reddit posts I've run across of a lone developer hyping up their latest project, only for me to go to the github page and notice the last commit was 7 years ago.
If you're not familiar with the Gemini protocol, it's an updated alternative to Gopher, which in turn was an early competitor to the WWW back in the 90s. Gemini itself I can't speak to, but if you go down the list of gemini servers and clients on geminiprotocol.net, you'll see 404s, broken links, and expired certs galore. There was a flood of developer interest 5 or 6 years ago when the protocol was new, but everyone wandered away once the shiny wore off.
My recent foray into wiki software has turned up a few corpses as well. Wiki.js development seems to have stalled, and Pepperminty wiki has been abandoned for three years now.
And yes, I know this is because FOSS devs are often doing this on their own time for little to no money, so passion is the only thing driving them, but passion can only get you so far.
Besides loss of developer interest, community schisms can cause a project to sink. Remember what happened to Audacity? I think it ended up surviving but there was a real concern for a while that the forks wouldn't be as well supported.
All the FOSS offerings I can think of that are "too big to fail" have big corporate support, like the Linux kernel.
I'm guessing most of us are self-hosting as a hobby, and we can afford to risk a loss of support when a project is abandoned, but businesses don't have that luxury. That's why they use proprietary software.
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Youre missing a few key things here.
I wouldn't put much agreement toward your argument for proprietary here, because I've seen (and had to deal with) proprietary solutions being abandoned with no workable solution available, especially as the current generation of proprietary solutions require a license server or other cloud-connected back end in order to work.
So when they get abandoned, its just over.
So no, this argument against open source for business has no merits IMO.
I had to spend months moving our IBM3270 automations to a new product because the company we had a license with apparently went under (they wouldn't respond to requests for a new license when we needed to do a server upgrade.
It took so long because their proprietary BASIC like language had basically no documentation, and the scripts had been written like 12 years before. At least I moved us to an open source solution, which didn't have the best documentation, but I could just look through the source to get the answers I needed (and had to program in some missing functionality).
I'm still sad that I wasn't allowed to contribute to the project due to our government contracts. At least I could contribute to the python wrapper I found for it to make it Python 3 compatible.
I'm not saying proprietary software doesn't also have problems, just that FOSS has problems unique to it that are rarely acknowledged.
Everything I implement at work is open source because I don't want to wait for a purchase approval. But I'm also practically the only one interacting with those systems, so I'm the only one who's affected if something breaks.
None of the issues you've noted are unique to open source though.
And most of the problems actually have solutions with FOSS vs proprietary.
Just to say, though, I feel like 99% of the software we deploy is open-source for that exact reason. Projects generally start out small, where you try to evaluate some concept. You're not gonna spend months to go through the purchase process of some proprietary tool, if you can help it...