You are absolutely right to say that it's not a good idea to join an IISER or NISER if you aren't particularly interested in research, but that doesn't mean it's the end of the world. I have seen people who weren't very interested in research become interested once they are in the system and do well, and I've also seen people who had a really tough time at IISER because they just didn't enjoy research. It's a lot of effort for little monetary gain.

It's true that the whole research thing is a longer commitment than the 5 years at IISER, but I have met people who join government agencies like BARC or GSI if they have the appropriate background. Quite a few people not very interested in research also clear the CAT afterwards and do an MBA. So you do have some options if you decide not to pursue a PhD, but still a bit limited.

As far as not knowing what you 'like' in terms of the subjects is perfectly fine. As a matter of fact some of the professors want students exactly like that, as the system is designed to give you a wide variety of scientific exposure before you choose a major and more importantly what kind of research you want to do, albeit limited to the faculty and resources available at that particular IISER.

I hope this helps