There will be transaction fees - easy like that.

People who already had mining hardware will stick to them and get the transaction fees - the hardware should have paid for itself already. There are already huge bitcoin farms. Most of them will be profitable with transaction fees.

Some will sell their hardware or shut down their hardware due of el. cost or they get damaged and not replaced. Difficulty will go down but stay high cause of ASICS going to be much cheaper.

Most of the big farms will stay alive where electricity is cheap or free. Maybe Asian, may be Afrika or even Dubai, when there is big money involved. Everyone else will be sorted out. Also possible: Solar Farms in Desert. Cheap area, free power, Sat Uplink. Whoever has feet in there will just grow more everyone else will starve.

It's an overall problem of BTC (one of many) - in the end only some few entities will control Blockchain = compromised = end of BTC. Just one of many problems which will prevent BTC from being successful.

Other Coins have much better technique and are more suited for future use.