It says that there will only ever be 21 million Bitcoin and the current BTC supply is almost at 19,094,150. What happens when its limit reached? will BTC price goes up instantly due to scarcity? is it the doom for Bitcoin since there is no room to mine? or maybe it will likely never reach 21 million?
Mining the last satoshi might not happened because the difficulty might be set to very hard at that time while the price is still the same.
It'd eventually happen, regardless of the difficulty, as if the difficulty is too high then the hash rate will decline and therefore reduce the difficulty as a consequence. What you're suggesting, the idea that the last satoshi might not be mined, is suggesting that the blockchain will be halted, as there is no way it can continue without miners being rewarded the final rewards.
It will be unprofitable for miners to mine that last satoshi if they will consume too much power just too mine a few satoshi but this will be a different story if the price of Bitcoin grow astronomically which no one can predict because there’s too many factor to be considered to come up to the estimated amount in the future.
I don't see a case where miners are mining for only a few satoshis, as there will still be the transaction fees which will be higher than the last remaining satoshis.