While there's bound to be other (more efficient) solutions, blockers could start filtering by the content returned. This sort of behavior leads to an arms race, with ad blockers ultimately losing (cost to detect >> cost to circumvent).
see https://en.wikipedia.org/wiki/BPCS-Steganography
The networks would be able to utilize many cores in parallel to mutate the content, but the blocker client would have to run it's detection on mobile devices.
The more processing the blocker client must perform the more burden it places on that relatively underpowered mobile CPU which increases latency. Also when you get into algorithmic detection you have to start considering false positive ratio because any noticeable false positives will break the user's expectation causing them to eventually lose confidence in the client's utility.
As you rightly point out there may be both vectors for either side which neither of us has thought of though.