Admittedly they were a bit cheaper back then (but this will one will be too next year)...
Also, it's not like this is a hypothetical question, they've been around for decades. They do have a track record you can refer to, instead of just blind faith.
See: Garmin nüvi.
It's not as though my cell phone will continue working forever. Nest discontinued Nest Aware. I've gotten bitten by this exact phenomenon more times than I care to admit.
I don't care about Garmin's reputation, it's simply a fact that having satellites talking to specialized devices requires a critical mass of subscriptions. There's a chain of vendors that need to all be on board to support all the hardware that keeps those devices online and updated, and at some point they will be discontinued. Probably sooner rather than later, especially when plenty of new phones make the functionality here redundant.
You'll have to elaborate, that's a wide product line. And they still sell map updates for many Nuvi devices: https://www.garmin.com/en-US/p/1456/pn/010-D0743-00/#devices
That at least bodes well for long term support.
I suspect that subscription supported devices will actually get better support than standard Garmin products.