But the ISP isn't semantically involved in the communication, so it's technically trivial to build stuff on top that prevents manipulation--namely, to encrypt everything. Also, it's not an economical problem either, as ISPs do get paid by their customers, so there is no need of any sort for them to be semantically involved.

You and I can establish a TLS connection between us through our respective ISPs, and the ISPs have no clue what we are talking over that connection, nor can they change it. The worst thing they can do is to drop the packets.