Good point! But that's "1/f" (frequency) liquidity. If the true value of bitcoin is as an instantaneous transfer (and it is), then the chance of a shadow wallet emptying in between the time you received and when you cashed out is vanishingly small. The cost to insure against such fluctuations while it gets multiple confirmations is your "transaction fee".

Bitcoins shouldn't be considered an investment -- the business about hoarding them like taxi medallions because they only go up in value is nonsense. Their value is limited by the low transaction fee for another blockchain. But you're right, you'd have to factor that in if you were actually investing.