There is an air of unreality about the OP’s assumptions.
Markets just do not operate in the way supposed. General expectations are often wrong. They are often about the stock market, and they are often wrong about money. The reason is that a market move is generally complete by the time the majority wake up to it. Then there is no one left to wake up and hence no one left to continue pushing it in the same direction.
Look at the price action of gold since it was allowed to trade freely. Since Apr 2001 it has risen at a compound rate of almost 20% pa. In the same period the nominal yield on 10-year T-notes has fluctuated between 3% and 5.3%.
Clearly the forthcoming appreciation of gold in 2001 was NOT generally expected. If it had been generally expected, it would have been bid up immediately to its present-day value, subject to a modest discount (say 4% pa). If this epiphany had occurred in Apr 2001 then gold would have risen overnight from $255 to over $750.
Gold, in fact, tends to take a breather just when the majority becomes convinced that it can only rise in price.
All this, of course, is well known to contrarian investors. What does it suggest here?
It suggests that the price of gold, even when gold again becomes money, is likely to be bid up by savvy speculators long before the reasons for the move become clear to most people. Smart investors will start hoarding gold when they detect the first signs of imminent productivity growth on the horizon. The rising purchasing power of gold will baffle most people until it has already been underway long enough (say a year or so) for the reason to become plain. Then they too will seek to hoard it. The smart investors are happy to unload their holdings onto the johnny-come-latelies.
In other words, the expectations of “price deflation” (pardon the expression) will be out of sync with the reality. So we don’t have to worry about the loan market drying up, at least not at a time when gold is actually undergoing rapid appreciation.
Furthermore, no one should whinge about a shortage of money even if the loan market does dry up somewhat due to a false apprehension of imminent appreciation. The reason is that borrowers, too, are not a homogenous class. The “average” rate of return on an investment means nothing. The most profitable investments (those with an exoected return higher than the market rate) will be funded and the others will miss out.
The OP’s assumptions, then, are too artificial to discredit gold as money. Indeed, gold would serve its purpose admirably even if all future gold mining were banned immediately on environmental grounds. A static gold supply would be superior to a dynamic supply, at least if non-monetary uses are disregarded.
In the event of extraordinary demand for a non-monetary use of gold (e.g. the elixir above), or a supply glut due to the discovery of a cheap method of alchemy, gold would be unsuitable as money and an alternative commodity-currency would be needed. But there is little imminent likelihood of either.