Yes, like many things in life, there isn't a simple answer to this question.
I know there are many cases in which it's difficult to take a decision. Yes, if it is the only subtitle available I think you can relax your rules. It's always better to have a subtitle with OCR errors than no subtitle at all. But there are also some clear cases in which deletion is the only reasonable action. For example, with this film, "The princess bride", several uploads add nothing to existing subtitles. No, sorry. They add a lot of errors. It is not that they aren't better, they are even clearly worse than the existing subtitles when they were uploaded. This is a clear case in which the upload should have been rejected. Or the file should be deleted now. Don't worry I'll report when I find such cases.
You know, the end-of-line convention is one of those funny, incomprehensible things. It is an example of how difficult is to find an agreement between different people. It is like a modern Tower of Babel. Something as simple as representing the end of a line. If you try to find some universal format you could say: "ASCII text". And then you discover that even then 3 different groups of people came to 3 different ways of representing end-of-line. There is a subtitle which is the same as another older one with a different end-of-line convention. Why not another upload with '\r' (Macintosh) end-of-line? You can't negate this is absurd.
I still think there should be some rules. I don't know what. But a minimal set of rules that everyone can agree upon.
I still think you should store everything in UTF-8. Either forcing uploads to be UTF-8 or asking to send encoding information or automatically detecting the encoding. Even converting everything to UTF-8 would be nice. But I'm aware that when you have 3 million files it is not so easy. This is why I think it is better to be less permissive with some things, before the mess is too unmanageable.