The centralized UX could censor transactions like you say, but the barrier to entry is low for another UX, built on the same decentralized data, to emerge and challenge the centralized UX. That's because the data is available to everyone.
I don't agree. I think excellent UXs have a super high barrier of entry. Look at apps such as Google Docs, TikTok, etc.
A company could build an UX without even thinking about being decentralized and users will care about the UX but not about "decentralization politics". If an excellent decentralized UX is built, you just copy the interesting parts and add you proprietary ones, sell you brand and success!
TikTok - not just a UX, but a massive social network, again with lots of data.
In a decentralized world, there will be a decentralized social network like TikTok. The UX would interact with that social network; it wouldn't create the social network itself. To give another example, it's as if Facebook's social network was open to anyone to build a UX on top of and interact with. Much lower barrier to entry.
Now, Uber^2 publishes a great mobile app, the mobile app is obviously centralized and they can decide to use or not and how to use the decentralized mechanism because they are the winners as the UX.
Isn't this obvious?
Your UI (e.g. mobile app) is centralized so the best UI will eventually win because most end users care about UX or will enter this interface because they see a well paid ad. If this UI (e.g. a decentralized search engine interface) wants to censor some content then it will win over the decentralized protocol behind it.