What is the difference between investing with bitcoin vs investing with fiat? One is closer to ideal money than the other and thus will win. They may work in harmony up to a tipping point where the preferred market instrument will take over due to higher transfer utility. You seem stuck in thinking that people cannot invest or will not invest with bitcoins. If the breakthrough is significant people will want bigger gains than what they would have been getting with btc.. if not then the breakthrough isn't big enough to justify large capital investments. Just as there is demand to hold coins over investing there is also likewise demand to lend bitcoins to high quality borrowers.
This brings us to the question of what is ideal money?
Ideal money is not something that just holds its value over time. Money is ultimately a signalling system. Like the nervous system it's function is to coordinate independent actors directing their activity appropriately across the fitness landscape of the world and the economy.
Bitcoin could certainly be an ideal money but it would be ideal money for a very specific society. Bitcoin would be ideal money for a society that was entirely or mostly static with limited or no technological progress and a stable population. In such an environment an entirely fixed supply of money would be ideal. It would allow stable transmission of value facilitating division of labor and coordination across the economy.
If we imposed a system like bitcoin upon a society with growth potential people would still invest, but they would not invest appropriately in response to economic opportunities. They would under-invest because a fixed supply of money creates a strong incentive against risk. With an utterly fixed money supply you receive a percentage of future economic growth simply by doing nothing. Only the most promising growth opportunities would receive investment and those with more marginal returns would be ignored. A bitcoin monetary system thus creates a strong vector against growth or in bitcoin lingo a HODL mentality.
Similarly a currency build upon the expectation of eternal exponential growth also runs into difficulties. Economic growth is unpredictable as it requires knowledge formation. Nothing grows exponentially forever there will always be periods of consolidation with limited or even no growth. Fiat does not allow for such periods as the entire system starts to collapse in a prolonged zero growth environment. This collapse is why society introduces artificial growth via government spending, central bank purchases of bonds, wealth redistribution and guaranteed incomes. All of these interventions are inefficient and inhibit growth.
It is impossible for any monetary system that requires either exponential growth (fiat) or is optimal in zero growth environment (bitcoin) to appropriately coordinate economic actors to a dynamically changing economy. To be ideal money a system must be adaptable across all future economic landscapes facilitating growth or inhibiting it in response to underlying economic opportunities.
It may be possible to approximate ideal money with some form of dynamic algorithm that changes in response to economic activity and population size but I am skeptical for this represents an attempt at top-down central planning coordinating the money supply via growth measurements or economic models. If we allow for dynamic competition between multiple currencies, however, we eliminate the need for accurate top-down omniscience. As long as the cost of converting from one currency to another is low future money does need to be a single uniform system. With a multitude of currencies to choose from (each based on differing fundamentals) actors would be able to move between currency systems depending on their economic needs and opportunities. This movement would alter the fundamental value of these currency options. The aggregate growth vector would become dynamically adaptable to all scenarios and thus appropriately coordinate actors across the broader economy (fitness landscape).