It's been like this for decades. I used to 'cling' to my perpetual licenses, until I realized OS updates would invariably break. Could be 3 years, or 6... but eventually they'll stop working.
Decades is a long time -- maybe 10 years. I believe even Office 2007/8 and 2016 still work on the versions of MacOS available at the time and a several later.
Note the issue here isn't that a future OS update breaks an old version of the software. The issue is that a legally licensed and working software on a legally licensed and working OS will stop working on an arbitrary date.
Life is too short to keep pushing against the tidal wave. I'll pony up for Microsoft365, or find an alternative solution (i.e. native Mac productivity suite). You're not going to win against the will of a corporate empire.
I suppose we could just all capitulate and accept descendence into peasantry...
I agree however it does make sense to take a strategic approach to our personal (or corporate if you run those things) supply chain. I am okay with subscriptions where they are inherent: internet, search, etc. Or even a reasonable trade-off where I could own but there are many competing alternative that I could switch to without much cost (e.g. electricity or e-mail hosting as long as I own the domain and have local copies of all my e-mails). However, when it comes to tools to create and manage my IP (e.g. Word documents) then I insist on owning the tool. As in, nobody can take the tool away from me or raise the price on me unilaterally.
What caught me off-guard in this case is that a working installation could stop working. A clear violation of vendors' oft-claimed falsehood that people who legally purchased/licensed software have nothing worry about from license protection schemes.