Does a full rewrite of the IDE, in an entirely different language to boot, define a "separate product"? That's hardly exactly the same...
It's a bit like saying that Windows 10 and Windows 8 are the same product because they fulfill the same purpose... or that this year's iPhone is the same product as last year's... or that new car being the same product as last year's. You're not going to get free upgrades for those just because you bought the one from last year. (Well... Windows 10 for a while after its release aside.)
It's a different product because it's different, in make and features, but not necessarily in purpose (that would make it a different product
line).
It's not a subscription because you can still use older versions of GameMaker -
yes, even GM 8 - if you have a valid license. The old product doesn't stop working just because there's a new one (some notable exceptions like GM 7 aside due to its dependence on the now-poofed Softwrap). You're not
forced to upgrade.