Currently, the consensus is that after block 6930000 (approximately on October 8th 2140 AD) - when the last new 0.00000001 BTC is generated via block rewards, Miners will be sustained via transaction fees paid from the network users.

That is of course, theoretically subject to change. For one, we are confident that the Bitcoin network by then would differ so much from what it is today we no longer recognize it. It could be possible that miners and users of the future would opt for a change in the inflation structure with overwhelming consensus.

It could sound near heretical at this moment but we just don't know what would happen in the future. After all, we are talking about what would happen 4-5 generation from now.

You can read more from this Bitcoin Wiki article that explains in more detail.