when bitcoin block reward was 50 bitcoins the miners were earning about $5 per block they found. then time passed by and it halved to 25 bitcoins and miners by that time were making about $20000 per block! then it halved again to 12.5 bitcoins and by this time they were earning 60000 on average which also went as high as $300,000 per block (12.5 + ~3BTC fees * ~20000=310k).
now you are asking what will happen after about 100 years when the block reward is so small that the total circulating supply is close to 21?
the answer is you can extrapolate the price and see how that small reward is going to be worth a TON.
Good point. If Bitcoin's price becomes worth more over time, then miners would keep supporting the blockchain for many generations. A small block reward must be worth a ton of money, otherwise there would be no reason to mine Bitcoin at all. Once all 21 million coins are mined, things will become interesting as miners would only earn via the network's TX fees. Considering that so many Bitcoins have been lost and forgotten, the available supply is much less than we've imagined.
This means that Bitcoin will become an extremely rare cryptocurrency in the future, that's sure to make you millions of dollars if you hold on to it for a long time. I believe that once all 21 million coins are mined in the future, we would be dealing with Satoshis instead of a quarter of a Bitcoin, a half Bitcoin, or a full Bitcoin. But everything will depend on Bitcoin's price for sure. Just my opinion