And for non-malicious content, you can drop file above the upload max limit before user wastes time sending the file.
Because now a vulnerability in PHP affects practically 100% of the users. But with more diversity, it would affect only a part. And if one runtime proves insecure more often than another one, organizations have a choice to move to an interpreter that better fits their security needs.
I bet that takes a week at most.
I'm really not sure how you mitigate this issue, other than serving static HTML and reducing the attack surface to cut out dependencies like PHP.