My Tab Pro 10.1 (£450 retail price, mind you) has never received a single major update from Samsung: it's still on Kitkat. If software support is of the highest priority, I usually suggest going with somewhere other than Samsung, which is otherwise a shame because I think they produce the best Android tablets around.
That said, it all depends. It will be a long while before Kitkat share numbers drop enough that developers can't afford to stop supporting Kitkat-based devices, so apps will keep running on this old OS for a long time and it's still functional enough. The major risk are security updates. You may want to avoid doing shopping or online banking, for example. But if you don't do anything critical of the sort, then it might be of little practical relevance.
Enjoy your Tab S2. I believe Samsung is trying to be a bit better with updates and I'm pretty sure it will get at least Marshmallow.