Once every bitcoins are mined, we will just continue using it as currency.
Does BTC just end? Will transaction fees be enough to incentivize nodes to keep working?
Only btc mining will be ended. By that time there will be more powerful sources of renewable energy and more powerful hardwares, so I think there will be alternatives to miners to create the transactions and keep the network operating.
I believe governments could keep the nodes working, through a system which people would only need to pay for maintenance, what means cheaper costs on transactions.