"rather about systemic risks in using any sort of proprietary software whatsoever"

There actually are under common distribution and licensing models. I used to think Stallman et al were vastly overstating the situation. These days, I think he was mostly right based on what companies did & do. I'll give you a few data points for your consideration.

1. Privacy/security. Proprietary vs open-source is false dilemma given first, secure system was a proprietary system (Burroughs B5000) that shared source with users. You can share source for vetting, local builds, or whatever while charging for it. Yet, most software comes as a binary where devious things are easily hidden. Many easily prevented 0-days and backdoors (esp undocumented FTP or SSH) have been found in proprietary systems over time. Even firewalls per Grimes' regular assessments. People are running out of room trying to find all the places Windows 10 is tracking users. They can't even turn them off. Leads to next risk.

2. Control. This is really most important. A fully, OSS product lets you use it however you choose. A proprietary licensed product, esp if not perpetual, can arbitrarily change how your product is allowed to be used later down the line. They can legally shut you out of certain benefits. This is getting common with app stores, DRM restrictions, games. I used to get games I could use permanently. Now, I often have to get online to access profile for even single-player games. The vendor, despite protest of users, plans to take service down after some time to force us to buy more expensive stuff. They also put ads in there because we can't turn them off so why not. Control is very important and many major companies are abusing the fact that "our" devices/software are actually "their" devices/software we merely get licensed to use only how they want to. Future-proofed against this with FOSS.

3. Lockin. Proprietary vendors often use obscure storage formats or communication protocols to make it hard to extract your data. They use custom API's to reduce portability. The result is that, after you build on them enough, you're effectively stuck with them since a move would cost exhorbitantly more than just paying an obscene licensing fee again. Such lock-in lets companies effectively stop innovating and benefiting their customers while their customers are powerless to do anything since the business, its apps, or its data just go bye-bye. Open storage, protocols, and API's can mitigate this but they (esp Microsoft) have nasty habits of subverting those with extensions or undefined behavior. So, FOSS clearly has a win here as you can just use the source itself to get off the platform if you want or pay someone to improve it.

4. One company I know specializes in proprietary hardware they sell but with FOSS software. In a discussion, the lead engineer told me he refused to use proprietary in their products since he was burned badly by one. The issue is the right to inspect code to debug and fix the dependency. You don't have it with proprietary & the vendor might not give a shit once you've already paid them. He (and FOSS advocates) argue that the complexity & bug-rate with modern software make it imperative to have source to ensure anything you build on it works correctly now and later.

5. Legal risks. No secret that licensing is often a minefield where compliance can be tricky. Vendors make it hard unnecessarily & overcharge. Business Software Alliance represents them telling customers' employees they'll get bounties if they snitch about instances of this then suing the crap out of small and mid-sized firms that didn't pay Microsoft, Oracle, etc enough. Big firms stay patenting software stuff, even cut n paste is Microsoft's haha, that they use to sue any competition or even users cloning to escape a bad platform. Recently, Oracle's argument that API's are copywritten essentially says nobody can make an alternative that's backward compatible and Oracle's users should be legally forced to be stuck with them. All these risks, except patent suits, are nonexistent when you use FOSS software. Plus, working with predatory companies seems wrong on principle.

6. Abandonware/bankruptcies. Company gets tired of supporting something or goes out of business. They can force you to keep buying something while basically not updating or supporting it any more. Lots of games and old apps in that status that are still fun/useful today but have to use emulators due to no source or legal restrictions. Impossible with FOSS as someone can roll up sleeves and code.

So, there's some datapoints that have and currently are burning up companies that invested in proprietary instead of open solutions. The major FOSS techs from 10 years ago still exist in some updated form with many others in development. Clearly systemic risk on one side of the equation with very little on other. What you think? :)