Further, I believe it likely that massive adoption will lead to huge numbers of nodes - naturally. Every common consumer has little reason to run such a non-mining, fully-validating wallet.
But I want to run a full node, and with huge blocks I don't even have a choice.
(SPV is a middle ground but not sufficient)
You are forcing me to trust someone.
In fact you are forcing me to trust the miners who are driven by incentives that do not align with my own.
In the future they may change the rules and I wouldn't even know, and if I did know, there would be little I could do to about it.
This myth that non-mining nodes don't matter is the most dangerous the big blockers are forced to peddle, simply because it drops out of the big block argument as a necessity.
The ability to run a full validating node is not optional, it is a requirement, and any fork that prevents this functionality fails the community.
We must not compromise the security of Bitcoin to help it scale.