Nothing is going to stay in some fixed microcosm when it is projected to grow. Satoshi himself said that Bitcoin was going to be big or nothing, so there was no vision that Bitcoin would remain some niche experiment as it, of course, had to start out small before it could become bigger.
Again: once, while discussing future user base growth, Satoshi considered 20% per year (doubling every 4 years) a "crazy" rate of growth. That would be consistent with his chosen block reward schedule (halving every four years). With a "non-crazy" growth rate of less than 20%/year, the USD value of the block reward would have deceased with time, forcing the system to transition gradually to fees.
Surely he was too smart to risk actual predictions, but on the other hand he cannot have expected the price to rise by a factor of 10 every year for 5 years. So the USD value of the block reward too increased by almost he same factor, instead of decreasing as he may have expected.
The price rose so fast because of speculative investment and trading, fueled by predictions that it would one day replace VISA. That surely is a use for bitcoin that he did not expect. If there is something that the world did not need in 2009, and will never need, is another penny stock. He would not have bothered creating bitcoin, if he knew that it would turn into that.
If he did not expect that to be part of the behavior, then he was completely stupid about markets, and I rather doubt that.
Once the possibility that it can someday replace VISA exists (or alternately function alongside VISA at a similar scale), people will speculatively trade on that possibility. Markets do not function in a manner where the value is in any way proportionate with number of
current users. To the extent that value is proportionate with number of users at all, the basis for that proportionality is
expected future users, a quantity that will certainly be highly volatile as current developments influence market consensus expectations of future outcomes.
How people on a trading thread could be so completely clueless about that is a mystery to me.
Anyway, block rewards will not necessarily increase nor decrease in USD value, it just depends on future market conditions. Either is possible. I certainly expect that this upcoming halving will see a decrease in USD value, but I can't be sure of that of course. As for future ones, nobody has any clue, whether they claim to or not.
Regarding signatures, I agree with JorgeStolfi that the simple measure of just limiting transactions to some reasonable size (at least temporarily) is easy and sufficient.
EDIT: He seems not clueless about markets, so we can rule out that he didn't expect and accept that speculation would occur
A rational market price for something that is expected to increase in value will already reflect the present value of the expected future increases. In your head, you do a probability estimate balancing the odds that it keeps increasing.