You can yes, but the problem is: Once you buy some new device you're going to want to integrate it and that is when you start running into problems without upgrading.

But yes it will work forever in terms of it not being cloud-connected. Of course with the exception of integrated devices that don't have any local access.

But yeah it's a pain if you don't do it monthly because you have to manually go and find all the intermediate changelogs and read through them. Or you just pull the trigger and see what breaks and then google how to fix it. Which is what I tend to do.