As far as I understand (and from an extremely cursory google search), (crypto) yield farming is described as "An investing strategy". Not as a bonus on top of something else. i.e. the "bonus" is the only point.

Your argument (again, afaik) was that yield farming should be likened to bank sign up bonuses.

If I follow that argument:

If someone were to tell me in the non-crypto world that there was a new "Investment strategy" and when I asked what it was based on, the answer was that my money would be used to pay people to open bank accounts to get the sign up bonus and that part of those bonuses would trickle back to me (making this, in some sense, "an investment strategy") -- I would run away.

But maybe I'm taking the analogy too far :-)