Twilio and their users are the victim here... Twilio having KYC would not solve the problem, they do not the own the numbers the expensive texts are being sent to. Twilio should just enable their anti fraud systems by default (by the way this is no panacea, like every other anti fraud this is a cat and mouse game, there is often no clear way of telling that a number is premium rate, and many carriers are in on it too and use normal mobile number ranges).
As a programmatic telephone company, they're a possible (but not really probable) base for fraudulent spam calls. With KYC, and the fact that Twilio requires you call from a number you control, fraudulent calls would be easy to trace back to a person who could be charged for the calls. Much better than status quo, where it's very difficult to get to the originating phone account, and if you could, it's probably not really connected to a person.
Surely the overheads of any useful KYC are way too high for this to work? And basically nobody in this industry does KYC, so how do you propose that would meaningfully affect their interconnects?
E.g. why don't they have KYC controls during account opening? Because it would reduce the # of people who open an account.