So users would be on up-to-date version of GMS, and learn to use new features or changes as they come to be.
Generally this is why subscriptions were made for. For those who are 25+ years old, they for sure remember, that most of paid apps have bigger release every 1-2 years (eg. Photoshop, Corel, Office, Sony Vegas, Partition Manager, Delphi, etc.). And of course except of most professional users, others paid for one version, and didn't updated for decades, even if there were interesting updates, because as long they were on same OS, and those apps were working, they were enough for them, and paying again for same software with one or two additional features was too expensive. In fact for companies some software and hardware is also expensive, that's why they... lease things. But why would companies lease things, and pay loan interest, while companies can do it on their side, without that cut-off for banks, and instead of releasing one big update every 1-2 years, have more continuous cycle, and customers can get new features and bug fixes more often? Of course, in long-term for non-professional users subscripts are more expensive, but only if we assume, that someone was buying some software and using it for 5-10 years without update.
That was of course possible with GameMaker (some are still using GM8.1 or GMS1.4), but this is true only for Windows exporter. If you use HTML5/Android/iOS/MacOS, then APIs for stores and software changes at least once per year, which means that even with perpetual licenses, those became outdated in short perspective - and GMS need continuous updates. It's not that GMS changes, it adapts to other software around.
IMHO, there's only one case in which subscription is more expensive - if one used Windows exporter only, and was making executable at least once per month (so at least 12 game exports per year). That's when subscription is more expensive than perpetual with no doubt. For all other cases - it's comparable, same, or cheaper. I will get into my forties before my subscription from current licenses runs out (having 34 now), so I won't tell a bad word about that model (and I'm spending monthly at least 40-50$ on different subscriptions on other applications and stream services). In fact, even internet, phone and some local bills are kind of subscription, as for me they are always same every month
And that's why I can understand this whole change is a problem for students, or for those who doesn't started their first job yet.