If the next iOS is not supported, that does not mean that you can´t use suddenly many apps. Sure, slowly some apps will demand the new iOS version, but in most cases you can just stay on the older, even download the old app version, for a long time. Our iPad mini 1 is on 9.3.5 and still supports all apps that are on it. Our iPad 2 is on 7.1.2 and there are only a few apps I wanted to install that demanded Ios 8 or higher. So, you can use the device quite a few years after the last possible iOS update. iPad mini 4 will be fully usable until 2020, I expect, an be mostly usable much longer.
Sure, I never said you will not be able to use it. This is why I said "it may become frustrating", and not "it will become impossible". All I meant is that, as a prospective buyer, you have to think about this a bit in advance. If your use does not require many Apps, or the research you have done makes you feel confident you will not be hit too hard by a lack of iOS support, then fine. Otherwise, then better to figure it out early, before purchasing, than later.