Letting the ISP inject stuff is the threat model, and it has happened in real life. https://www.theregister.com/2015/03/27/github_under_fire_fro...
Every connection that happens over unsecured HTTP is a connection that can be hijacked by middlemen for nefarious purposes, both for spying and for attacking.
Encrypting as many connections as possible is a form of herd immunity. Encrypting even unimportant "marketing" websites protects everyone on the web by raising the cost of interception and lowering the benefits.