Well, you ought a touch screen device and the forerunner is button operated. That's a little more than a software difference.

The 945 is larger than the 245, and it accommodates a larger battery and barometer, neither of those are software.

Fenix has a sapphire screen option, most other devices use Corning Gorrila glass 3.

Yes, the devices have the ability to do more with the software. And the elevate sensor array is used on the majority (all?) of the 2019+ models. They also use the same Sony chipset I believe.

But there is more between tiers and devices than just software.

And a subscription is ridiculous. We pay a fair amount for a solid device as it stands. If you want the features of a device, buy it.

I think it's probably common for people to go from entry tier to mid tier / from basic fitness to sport specific, as their needs change. Same goes for transitions to upper tier, or those that go down a level.