That's all well and good until whatever business you run competes with the DNS provider or offends them in some way and they stop resolving your domain name. You're not going to say, "well, people can stop using Google's DNS if they really want to use my service".
But love them or hate them, they're vital for many companies' and websites' revenue. If big tech companies started implementing anti-ad features by default, there would be chaos.
That said, any anti-ad features need to avoid breaking things for users, and it'd be entirely too easy to break a simplistic DNS-based ad-blocker in ways that would end up hurting users. Client-side ad blocking can take more sophisticated steps, both to deal with anti-adblocking technologies, and to give users feedback and recourses if something goes wrong.
Those ad networks might even die off completely! And that'd just be terrible, really; however would people get by without being advertised at?
(Ideally you'd want the ad domains to not resolve at all, though, rather than resolving to localhost.)
I don't see how someone adjusting a service that they completely own and control to provide a more useful service would fall anywhere near a "line". That line is entirely drawn by the owners and operators of the service, and people can always choose to use a different service if they don't like that.
(That said, ad networks have sadly gotten sophisticated enough that DNS blacklists alone won't suffice, and could potentially break sites.)