I completely do not understand why anyone thinks the Masquerade pin made Dominion a better game.
As much as you might have enjoyed pulling it off, just in terms of game design, mutually enjoyable play experience for all, etc. it's obvious why the game was much worse off for having it than for not having it. Playing against it could ruin Dominion for players, easily. It was just an awful game mechanic that you guys just enjoyed exploiting, not some treasured part of the Dominion experience that made for quality gameplay. I'm glad it's gone.
I don't mind it being removed but it was an interesting thing that would show up once in a thousand games (because of being a 3-card combo). Not common enough to create degenerate games on a regular basis, but just barely common enough that long-time players will see it a few times, making it fun for analysis and stories.
That said, I've barely played with Adventures and later, so if Royal Carriage makes it a 2-card combo, then a fix was needed for sure.
The Masquerade pin was never a three-card combo. Any discard attack with KC and Masquerade worked just fine for the full pin, but even without a discard attack it was a highly painful combo. A Kinged Masquerade with an empty hand is enough to get your opponent down to 2 cards in hand, and the other 3 cards in his hand will be trashed, so it's still a brutal combined discard-and-trashing attack. Royal Carriage could perform a full pin on its own, that's true.
Most people also think you have to trash down to a deck of only KC-KC-Goons-Masquerade. That's not at all necessary, you can play any engine as long as you trash all your Coppers and Estates, and you make sure that the last card you play is a Kinged Masquerade. The pin was way more flexible and had more finesse than people say.
Any pin short of a full pin was bad because 1. your opponent gets to keep their best cards on each turn, 2. your opponent can often disrupt it by playing a single Masquerade, and 3. the kingdom has KC and Masquerade in it, and as such the strongest alternative strategy is probably very fast and hard to outrace.
It's true that there were several different 3-card combos that could do the pin, but they're still 3-card combos. It wasn't very common. Suppose there are 10 cards that complete the pin with KC and Masquerade. Then the chance of having pin in kingdom was very roughly 10^3 * (1/200)^2 * (10/200) = ~0.1%, so my 1-in-1000 guess was the right order of magnitude.
Edit: To be fair, I could be totally wrong on this, because the last time I was any good as a player was pre-Goko. But isotropic had KC+Masq appear together at higher frequency than today, so the 3-card combos should probably be _less_ common today if anything.