No, Kontakt is not a subscription, although NI is on the cusp of introducing more subscriptions for Komplete (as I understand it) - so according to NI on their own forums that will be an additional option alongside perpetual licenses. (For info: they released a survey where they asked whether you would pay a subscription to “always have the latest tools.” So you can kinda see where they’re going with that.)
In terms of the upgrades, it is somewhat like a DAW like Ableton where you pay for your original version and then you will be offered an upgrade price when a new version comes out which isn’t the full price. As an example; Kontakt is currently priced at £269. But it will be on sale in summer for £134.50. And then the upgrade (from a previous version) is £89, but this will also drop during sales season. So if you move at the right time it can be reasonably cost effective. So 2 versions of Kontakt standalone should cost maybe £185. As for how long that lasts, it’s really down to luck as to when you buy into it…
On timescales, at some point there will be a Kontakt 8, which you’ll be able to upgrade to either on its own or via a Komplete bundle. Kontakt 7 was released in 2022, K6 in 2018, K5 in 2012 and K4 in 2009. So you can see there’s quite a variety of timescales there K7 is already on version 7.8, so I get the feeling that Kontakt 8 will be around in the next year or two. That said, NI tend to support one version back for a reasonable time as many musicians they serve are professionals who will be running K6.
What a lot of people do also is buy Komplete, which is both cool and infuriating, since you get a mish mash of instruments, libraries, FX - some of which are old and some are newer, some of which are great and others less so. Then I get the feeling a lot of people just pick the things they want to upgrade.
NI isn’t perfect for sure, and it comes with some issues no doubt. But if it’s simply loading some sampled instrument libraries (that isn’t a sub and that you can sell) it’s worth at least exploring, even if you don’t go up to a paid option with them.