If Bitcoin adoption(and price) continues to increase as time goes, this should be enough incentive for miners to continue mining. And knowing that it's estimated that year 2140 is the year where almost all bitcoin has been mined already, that's more than enough time for Bitcoin to have enough adoption.
In the end, time will tell. And unfortunately we wouldn't be around in 2140 to see the results. All we can do is to have enough bitcoin for our children and grand children so they can reap the fruits of our labour.
If Bitcoin does not increase in price and stays the same price that it is now that is enough incentive I think as long as Bitcoin is worth something then it will be worth mining for some people because as you say block rewards do not end until 2140 after that I think the miners will go down because they will only be rewarded with transaction fees but that is over a 100 years away. In the end, time will tell. And unfortunately we wouldn't be around in 2140 to see the results. All we can do is to have enough bitcoin for our children and grand children so they can reap the fruits of our labour.