This is an interesting point.
My Pi-hole blocks an average of at least 50% of requests at the moment (and I’ve seen it as high as 75%). Current client-side ad tech means that all those requests are being made by web browsers. If this was replaced by server-side systems then those requests would by definition be made by the web host. Would sites be willing to pay for the extra bandwidth I wonder? How do Akamai charges compare to ad revenue? 
Server-side might mean rather less possibility of black hat and cross-site badness too perhaps, since the security model would all be centralised at the host and not delegated to up-to-date browsers with non-rogue plugins.
Interesting…
