However, I would contend that with larger user base even if the cost of running a fully validating node increases, the absolute number of full nodes would likely go up not down even if the relative number shrinks.
That does not make sense.
I think what he means is that with more people using the network, the number of full nodes that run on the network will go up, but the ratio of full node to user will not increase. But I don't see this as such a big issue if users are allowed to run whichever part of the network that they wish or interact with economically speaking. For example, if I am geographically located in Region 1 Zone 2, I will run a node in Prime, Region 1, and Zone 2 as I do most of my commerce in those networks. Because data from a Zone is compressed when it moves into a Region (and Region data is compressed when it moves into Prime) the resources required for running the three nodes would not be that much higher than running a Bitcoin node. Now if you are a merchant I'm guessing you would want to run nodes in multiple zones (to accept payment in those zones), so the hardware requirement there would be greater, but you have an economic incentive to do so.
I have another question OP. When a transaction moves from zone to zone, there is a 'state transition' that takes place, correct? The transaction would go from zone to region to prime and then to zone (assuming the two zones are not in the same region) so the zone that the transaction enters would need to have some state transition that is not necessarily in a block but is also reversible (I suppose this could show up in a block in the other zone). What happens if the transaction is reversed in Prime due to an orphan or re-org? Does the other zone chain need to re-org its UTXO set?
Looking forward to hearing more, thanks.