What would be the implications of limiting Bitcoin transactions to fifty inputs?
The main concern with sigops is the quadratic behavior of the pre-segwit signature hash.  I believe specially constructed transactions with lots of OP_CHECKSIGS but few inputs would be similarly bad as transaction with many inputs.

Having additional limits also makes life a little harder for anyone working with the system; e.g. you have to be under 100k, AND under 50 inputs.

Ultimately the best solution is to avoid quadratic behavior like the new signature hashing in Bitcoin did.