Penalty for enemy introducing his religion

I don't quite understand why this is a bad thing, especially if you can't get Great Prophets anymore

It's not at all clear what you're trying to say here

Are you talking about an opposing religion converting all your cities? Of course this is a bad thing. You lose out on the founder belief. What makes you think you can't get great prophets anymore? You will still accumulate faith and you will still pop prophets automatically at the thresholds pre-industrial.

You might still get follower beliefs, but if you don't found a religion, you don't benefit from a founder belief, and a lot of those are very good.

its bad if you have your own religion you're trying to spread

if you have no religion and aren't gonna get one, then sure

the AI thinks this way too. they won't get mad if you spread to them UNLESS they are also trying to spread their own.