Weren't enterprises already on yearly contracts with licenses and support included? I know developer tools from Microsoft in the 90's had subscriptions, but I never dealt with Enterprise licensing back then. But, given some of the blanket enterprise licenses I did have to deal with, I always thought at that level it was always a subscription model.
I think the shift wasn't that the SaaS model is now new, but that the SaaS model was now also taking over consumer and small business accounts.
We used to buy Microsoft MSDN subscriptions, which got us constant upgrades of Visual Studio and other development tools. Those licenses were perpetual - you'd get a disk with e.g. Visual Studio 2007 on it, and you were legally entitled to use that version forever.
IIRC if we didn't think we'd need a new version anytime soon, to reduce costs sometimes we wouldn't purchase MSDN renewals.
I think Microsoft's licensing 20 years ago shows the prevailing view then was that companies wanted the certainty of perpetual licenses.
20 years ago, most businesses and consumers didn’t have reliable and fast internet. MSDN came in dozens of CDs/DVDs in a binder.
Back then, most people only had one computer and if you switched between Windows and Macs you had to buy a separate copy of Office. Now I can run Office on my Mac, iPad (and pair it with the same mouse and keyboard I use with my laptop), and iPhone. If I’m not near my computer but want to use Office on another computer, I can do it on the web.
There is also a lot more churn in the mobile space as far operating system and hardware upgrades that mean needing to update your apps. Despite bad blood between the two back in the day. Microsoft has been keeping up with the latest Apple hardware/OS initiatives since 1980.
Before that, they were running DOS on the client and Novell Netware on a server. Linux and “open source” has never been big in business.