I would it is a combination of metric-stuffing, land grab, and genuine concern about security.
Metric stuffing. Everyone at Microsoft is graded on "impact". All the EVP-types at Microsoft have their eye on boldface jobs, so they need a track record of massive impact. Beimg able to claim that they got W11 from X billion devices to Y is how theyll be judged. Another example is how in Azure, the only metric that matters is consumed revenue. That sort of thing drives behavior.
Land grab. W11 infamously makes the Start menu a billboard and has all kinds of usage data going back to the mother ship. If adoption slows, then Microsoft misses out on eyeballs, misses out on the ability to weld users to Copilot, misses the opportunity to earn money from ads, misses the opportunity to improve Windows by learning how people really use their conputers.
Security. Windows is embedded in modern life and although Microsoft gets a lot of flak, (and sometimes it takes a major beating to remind them of their responsibilities), they do want to elevate the security of users. They believe that W11 and TPM will give them a basis to really deliver stronger services. I dont know that they are right (eg if the #1 exposure for home users is ransomware, does a tpm help at all?), but I am prepared to give them dome grace.
Then again, I plan to use this opportunity to install Linux on my old PC.