I'm all for MS bashing and laughing at their incompetence, but was there really any threat there? I don't know anyone on PC who was interested in buying a game anywhere other than Steam in 2015.

It was specifically the release of Windows RT (Windows 8 on ARM) in 2012 that had people nervous that Microsoft wanted to lock Windows down long-term in the manner of iOS and Apple. Windows RT only ran code signed by Microsoft and only installed programs from Microsoft's store. It failed, and Microsoft let off the gas locking down Windows, but that moment was probably the specific impetus for Gabe Newell to set Valve on a decade long course of building support for Steam and the games in its storefront on Linux. Windows being locked down to the degree of iOS was an existential risk to Valve as a company and Steam as a platform in 2012. It isn't anymore.

Windows RT also drew ire from people other than Newell at the time IIRC. It was widely perceived as a trial balloon for closing down Windows almost completely. The first Steam Machines a decade ago were Valve's answering trial balloon. Both failed, but Valve learned and Microsoft largely did not... They haven't locked down Windows 11 to the point of Windows RT, but they're abusing their users to the point of potentially sabotaging their own market dominance for consumer PCs.

I buy games on GoG when I can, Steam when I have to. I have nothing against Steam, but they do have a near monopoly position on PC. Unfortunately the non-GoG alternatives are from even worse actors.

People feared that MS will make installing things not from the store harder. Like what apple is doing. It posed a serious potential threat. Given that MS had complete control over the Windows, DirectX and many other tools developers were using.

Sure, if they pulled Apple and locked everyone into only installing from Microsoft Store, Steam would have been in serious trouble.

I never installed Steam, nor I intend to.