I believe Valve's concerns went(or maybe go?) beyond just the Windows Store, and into "We believe Microsoft may become unable to ship a good Operating System in the future".
In a 2013 interview with Gabe Newell: "Windows 8 was like this giant sadness. It just hurts everybody in the PC business. Rather than everybody being all excited to go buy a new PC, buying new software to run on it, we’ve had a 20+ percent decline in PC sales — it’s like 'holy cow that’s not what the new generation of the operating system is supposed to do.' There’s supposed to be a 40 percent uptake, not a 20 percent decline, so that’s what really scares me. When I started using it I was like 'oh my god...' I find [Windows 8] unusable." [0]
The Windows Store probably was a part of it, sure, but looking at that quote from 2025, after having your SSD broken, your recovery unusable and your explorer laggy? It's quite bitter-sweet.