Of the Dominion players who have played 500 or more games, people might care more about a new set. But for people who aren't that into Dominion, but could be, having another set isn't a big deal. Having another set makes playing all cards games even more of a hurdle for new players than it already is, and Dominion games are plenty interesting these days without a new set. Having a better interface would help people get into Dominion in the first place.
Delaying the new set isn't going to get a new-players-ready version done meaningfully faster. A new set is an especially easy task, as has been demonstrated in the past here. Aside from occasional tricky cards like Inheritance, or cards that require unique interface stuff like Stash, it's all trivial. At the same time the new set is a significant draw for not-new-to-Dominion players.
It's moot as it's contractual. MF took forever to get Adventures up. So RGG has it in the contract that the expansions have to go up in a timely manner. This wasn't RGG saying "because that's more important than the user interface" or any such thing. It was RGG saying "man it sucked when the expansions weren't going up in a timely manner; let's make sure that doesn't happen." And since that's the contract, the expansions will go up in a timely manner.
Having an additional set is not a meaningful hurdle-increase for new players. They are seeing 10 new cards each game for a while any which way, unless of course they play all base set against a bot or fellow new player or what have you.
I find it strange that ShuffleIT would prioritize having lots of volunteers help moderate a tournament (for the most serious players) over essential user information like an official FAQ.
These volunteers you speak of, they are volunteering. The nature of this is that they do things they feel like doing. It should be no surprise that volunteers who are enfranchised players like doing things that enfranchised players like having done. It's not Shuffle iT prioritizing anything.