In the XP days we were still looking at fairly substantial improvements in CPU speeds, but the past decade has been really disappointingly poor on that front and people are fully aware that old PCs can more than happily cope with their software needs. It is much harder to force them to do something where the tangible benefits of doing it are reduced so dramatically.

Indeed - but I believe it was already difficult in the XP days. From personal experience, many of my non-technical peers initially saw no point in updating at all: They were perfectly content with their PC's performance, and had all programs they needed. Sweeping UI changes and old programs not working anymore was seen as significant risks.

MS had to rely on a lot of developer advocacy, an increased focus on the risk of unpatched security vulnerabilities (a valid point IMO) and a general narrative of avoiding "outdated software", (as if software had an expiry date) to increase the willingness to update. Even with that, in the end they had to patch XP to add nag screens to make people update.

Now, with less performance improvements, less developer advocacy (I hope) and more obvious anti-features, I imagine they will need to employ even more force to make people update.