Rare WalterBright L taken in that thread.

Sure, Unicode isn't the Platonic ideal of a character encoding. It has warts, legacy features, and.. and it is a universal encoding of all human writing. What an exceptional and incredible accomplishment.

Could you replace it with something better designed?

No. No, you cannot. You can in principle design something better, but that's a completely different, quixotic, and useless task.

It's also far from impossible to implement Unicode 'correctly', folks not only can, but do, routinely. It's extensively well documented, there's example code, it's just work.

Also, if your game plan for Unicode-D includes removing the most beloved and consistently demanded feature, emoji: then no, that person in particular is not capable even in principle of designing something better. That game has been lost before it began.