I don't think that bitcoin will be ever replaced by any other crypto currency as I don't see any competition to bitcoin at a moment and even in future bitcoin will keep on updating itself with the demand of people so its hard for any crypto to replace bitcoins.
For currently yes, but I'm sure someday there will be another coin that replaced the bitcoin. The main power of bitcoin is the first and most popular, most users, and supported other developers. But a big issue like taxes, illegal transactions, and the number of confirmation if not resolved then the user will run out.
Those illegal transactions are cannot be solved in bitcoin because of the anonymity feature of bitcoin and
that is why the criminals are using and abusing bitcoin but we can't avoid that because even in our fiat money that is happening and there still a lot of criminals that using fiat to do illegal transactions. Those big issues that you stated can be solved in just a few months of development for bitcoin but those illegal transactions are exempted
Personally, I don't think that criminals are massively using Bitcoin
At least, not big fish and not in the amounts in which cash is used for illegal operations, activities, and transactions. Bitcoin is far from being anonymous (it is pseudoanonymous, at most), all Bitcoin transactions (at least, on-chain transactions which still make up the total majority of transactions) are permanently written on the blockchain for everyone to see and analyze them. In fact, I'm really fascinated why people still consider Bitcoin anonymous, you can't even say that coin mixers are not used by government agencies as nodes to specifically track big Bitcoin transactions
Fiat is even more anonymous since there's no records on who held the paper money you're holding before you got it. It's not transparent and maybe that's the major reason why big syndicates haven't switched to using bitcoins yet. They use paper money since they can't use bank transactions as suddenly having millions of money to your account will raise suspicion.
Only way that you can trace who had the paper money before is by tracing the finger prints. But even then, the only one that you'll see is who had it very recently. Paper money is moving a lot and older finger prints are overwritten by new finger prints.