In case Bitcoin miners will only be relying on transaction fees then I think many of them will be getting out of business as there is a big possibility that the whole enterprise may not anymore be profitable (especially as compared at today's numbers). Now, I know that 2140 is quite a long, long way ahead of 2019 and I am sure all of here in this forum can already be gone to the "kingdom come" so personally this concern would not bother me at all. But then again, there is a possibility that fewer miners will continue doing business and then transaction volume have gone to the stratosphere already.