The key to what you're saying is "once BTS has reached its peak market cap." If it were to lose all growth potential, and we were still paying delegates in excess of burnt fees, then it would be unprofitable to hold BTS. I don't think we'll reach its full growth potential for quite some time though. While it's growing in value faster than the growth of total supply, investors have incentive to hold it even as the supply increases, because share price is still increasing.
Reaching peak market cap would mean basically that it's no longer able to create new value. If the delegates are doing their jobs, BitShares should adapt as necessary to avoid that ever happening, though growth should slow once it reaches market saturation.