So that's a partial answer, and my apologies if I haven't gone and found all your posts about it, but "if spamming costs more than the thing that spamming can win" is still there, right? The spam must be profitable or it will cease after the losses are realized. The argument cannot be "it is impossible to imagine a world where it is prohibitively expensive to do the wrong thing and relatively cheap to do the right thing" but something to do with the details in constructing such a world. Hashpower mobility, I assume, means that hashpower tends to get directed to its most profitable use and relatively quickly. I also assume that spam requires a cheap way to send many messages such that only a small fraction bear fruit in a big way, thereby covering the cost of the sending.

It sounds like your point (and I'm guessing again because I cannot read your mind) is that in order to cause the cheap hashpower available to spammers to be unprofitable, the imposition of PoW would make doing the right thing too expensive? That there is an imbalance between what hashpower is available cheaply for spammers versus hashpower available to the common Joe on a common consumer device?