bitcoin will have moved to pos by then.......... dont worry about it since you will be less worried by then too i expect

there are tx fees left to allocate after mining is done

Theoretically there will not be any big difference (like changing to pos or anything like that).

As butka said, transaction fees will be enough for the miners.

As halving is already going on, block reward is not that high as it used to be, and we didn't moved to PoS.

As 2140 is a very distant future, by 2021 block reward is going to halve again. That will be an interesting moment to watch. Probably price will go up as there will be less bitcoins going to exchanges.

Something tells me that it will happen in some popular museum, where visitors will be able to explore the evolution of money from seashells to the Bitcoin.

By 2140, Most small transactions should have migrated to using some variation of a second-layer solution like LN. Most on-chain settlements would be bigger transactions for which the parties will be willing to pay a good amount as fees for confirmations.

If sufficient distribution and decentralization could be ensured, we would have moved to a more open and free market with little scope for political motives determining availability of economic opportunities to people and countries. People would be much freer to monetize their skills if they choose to from any corner of the world. People would also be free to do more good for the poor. We will probably have a sharing economy.

Mining hardware would be advanced enough for anyone to contribute to the network without worrying about breaking even. The selfish motive would be replaced by a different understanding of what money is.

here is how the block rewards change over time

but anyway for mathematical purposes

year*  reward
2009   50.00000000
2013   25.00000000
2017   12.50000000
2021   6.25000000
2025   3.12500000
2029   1.56250000**
2033   0.78125000
2037   0.39062500
2041   0.19531250
2045   0.09765625
2049   0.04882813
2053   0.02441406
2057   0.01220703
2061   0.00610352
2065   0.00305176
2069   0.00152588
2073   0.00076294
2077   0.00038147
2081   0.00019073
2085   0.00009537
2089   0.00004768
2093   0.00002384
2097   0.00001192
2101   0.00000596
2105   0.00000298
2109   0.00000149
2113   0.00000075
2117   0.00000037
2121   0.00000019
2125   0.00000009
2129   0.00000005
2133   0.00000002
2137   0.00000001
2141   0.00000000

**based purely on current fees per block(~1.5btc)..
we could see the importance of reward:fee  flip to  fee:reward happening around 2029
where pools care less about the reward and care more about fees

the pools will continue to mine blocks because they get paid via fee's from transactions

*note dates are variable because blocks are not solved at exact times of 10 minutes(2016/fortnight)
for instance the january 2013 halving actually happened in november 2012 and the 2017 halving happened in july 2016. the next halving is ETA'd for May 2020(not jaanuary 2021)

Miners will still do the job and they will now mine to confirm transactions and they'll earn through the tx fees. Read these articles for better understanding.
What Happens to Bitcoin After All 21 Million are Mined?
What Happens to Bitcoin Miners When all Coins are Mined?

I have been meaning to ask similar question since but I didn't let it bother me much because we will all be gone by then. All I know is that this will push up the price of Bitcoin due to scarcity. I also agree more with the fact that once the majority of bitcoins are mined, the sum of the transaction fees collected in a newly mined block would be enough to cover for the mining reward. It will be interesting to see more views on this.

You are worrying much my friend. At least if all of the Bitcoin are mined then by the time that it happens then you are already rich.

Will miners continue to mine bitcoin after that? Will mining still be profitable? Will miners used in mining evolve to consume less electricity and make mining profitable with fees in the future? How will miners survive with tx fees? I doubt miners will still do the job if these questions are still questioned 50 years from now. I also think that instead of earning through tx fees alone, there would be methods invented to make mining profitable again before all bitcoins are mined in 2140.

The last bitcoin will be produced much earlier than expected, there are 4,000,000 bitcoins left, so that there will be changes in the coming years, but the consequences will be unclear