Faced with a choice between a vague future threat that might happen (an adversarial ISP or other MIM attack) and a certain future threat that will happen if we let it (incumbent gatekeepers locking down the Web), I'll take my chances with the former, and opt for less gatekeeping rather than more.
"That is because without Web Integrity, there is no guarantee that the site requested is being delivered as the site intends. For example, a browser extension could remove ads or modify content on the page."
See where this slippery slope is heading? We DO NOT want what "the site intends". We want to be in control of the content we consume.