If bitcoin proves to be fundamentally impractical for small, high-volume transactions, why couldn't it still have great value for use in large transactions such as real-estate, yachts, gold, etc. and as a secure store of value?
Because that would be an incredibly small market cap. One fact is the richest 3% control most of the money, but the masses (the 97%) spend most of the money that is spent.
"Exponential and power-law probability distributions of wealth and income in the United Kingdom and the United States" by A. A. Dragulescu and V. M. Yakovenko
I don't understand how you come to the conclusion that it would be an incredibly small market cap. It is already approaching 10 billion dollars, and its use as a store of wealth is minuscule compared to things like gold and offshore bank accounts. Why couldn't it increase in popularity for this purpose tenfold or one hundredfold?