That would still allow ISPs to compute the limited number of domains for which NXDOMAIN would need to be sent at any given point in time.
(Whether they'd do it is another story. The random pattern currently used by Chrome looks like it may still be easily detectable at the DNS-recursor level, so maybe the ISPs really don't bother beyond the simple NXDOMAIN -> portal domain replacement.)
The network operators enable this behavior. It would be next to impossible for it to be useful (ROI wise) if they didn't intentionally support it with access to their networks. It doesn't need to be an arms race, but we refuse to regulate or punish anyone in this space. We waste massive amounts of resources detecting and counteracting the hijacking services. The human (developer) cost is where the big waste is here, not electricity.
and the fight goes on....
I think the only downside is that you would leak some information about your system clock.