We don't know what will actually happen if all coins were already mined and we'll never find out cause we're already dead by that time. But if all bitcoin were already mined, bitcoin will still continue circulating and to be used for trading. No more bitcoin to be mined doesn't mean it's already the end. And I think during that time, bitcoin will be more valuable.
That is what many people have said, they have calculated the last bitcoin to be mined as year 2140 which you already said that many of us would have been dead by then, but I believe that by that time, the value of bitcoin will really increase because what will be circulating in the market now would be coins that has already been bought and no one will be able to get mined coin again which would make the value of bitcoin to become so expensive.
Bitcoin would be kind of scare by that time, and i think whatever investment we leave in bitcoin right now no matter how small it is, is what our generation them would bank on and always appreciate us for, which is why we need to keep our coin for long term investment that would benefit is and our generations.