That is true but only a very tiny minority of farm posts ends up in Trending or ever has

Right, because they would be highly visible and scrutinized, so it is an ineffective strategy that isn't used....

or ever has.

I would say this is very, very wrong. Some of the biggest farmers such as @haejin were on Trending every day for months or years. I can think of several others but I'm not going to name them because they weren't quite as blatant.

Now extrapolate that to where instead the farmers can block visibility. In fact we already have this experience (see below).

upvote comments

I'm all for that but comments also show up on Trending!

This wasn't previously the case but was changed specifically to give visibility (for community scrutiny) on large comment payouts. It was seen that farmers would hide their votes on comments.

BTW, this is also the reason that even spammers with rep < 0 aren't hidden when they have a > 0 payout. The idea is to ensure that all payouts are visible so they can be downvoted if undeserved.