> Though I'd pay £100 for it no questions asked.
Office 365 is a continually updated and enhanced product.
They won't provide those updates and enhancements to you forever because your £100 won't pay for them.
What you are asking for is their old downloaded/shrink-wrapped Classic Office product offering, which you can still get by paying a f themixed sum ($145 I think).
But even in the old days, your £100 bought you a single program with very few updates if any. If you wanted new features or even security updates (when this became a thing), you needed to purchase a new license, for another £100.
What if you don't want the new features or security updates? Well, perhaps it sucks, but the world of software is a far more complicated than it used to be and both the feature competition and security threats are steep, so no software vendor can afford to sit on and continually support an old version. Unchanging software isn't a product that becomes an antique like a 1959 Chevy. Rather it loses value over time as the world changes around it and it becomes less relevant.
Eventually, they will stop updating your purchased software, and if you want to have new features and security protections, you will need to pay again. In the long run, that cost will converge with that of a subscription.
This is why some people like open source software like Libreoffice as an alternative model, but as the OP highlights, it still needs an economic model to incentivize the enhancement of the software, and it would appear that the primary motivation of many users of the product is to not pay for an office software suite, not the higher ideal of a shared community software development model.