Mhhh, I'll have to do more thinking about this, but intuitively it feels like a larger network means stronger WoT.
You're right that we need to be careful tho.
Discussion
Probably need to look at hybrid models - trusted clients that offer some verification. Or some organizations that audit. Maybe some economic penalty (worked into the UX somehow). Even then, at scale there will likely be gaps where no one is watching and people are installing carelessly.
It's not evident yet but there are two layers of curation: relays and users.
The relay (app catalog) is the main curation mechanism. The Zapstore relay may decide to remove certain apps in the future, but that does not matter, the whole point here is having the ability to manage your own app catalogs. (In the alpha version right now this is already possible)
WoT will also be used to discover/recommend app catalogs.
User-level curation is talked about more because (a) it's innovative, (b) it's prominent in the UI, (c) in practice there's a single relay right now
