I wish I would know and imagine all. Yes there will be only transaction fee. So high for BTC and low for Monero. I cant really imagine how this will look in practice. I am sure we will soon read about some ideas. Two people that will want to trade will still need some place to met and decide price. Atomic swaps should sort of make real decentralised exchanges possible and very easy to build and run.

But my point was that Bitcoin will gain a lot. Monero will at least fro now gain a lot also, since some have troubles to acquire it with fiat.