As we all know BITCOIN mining will stop once the 21 million mark is reached.
Mining will NOT stop then. You have misunderstood something.
According to the information we have 19 millions coins were mined and now circulating.
Actually a big chunk of those coins is locked in wallets either owned by very strong hands (and will probably stay locked for many years to come), either lost forever. The real circulating amount is smaller.
The remaining balance is about 2 millions to be mined. Why it is not possible to mine the remaining BITCOINS before 2140 when the 19 millions mined in just over 10 years?
Bitcoin block reward is MEANT to halve every ~4 years. That's part of how bitcoin software was written, and it's also something that makes the coins price rise (not the only thing, still..).
You can easily make your own fork of Bitcoin, make the block reward logic change (maybe the difficulty logic too) and mine those remaining coins even in a day if you want to. But that will no longer be bitcoin and will most probably have no value.