how so?

because it is much harder to game it
upvaluing may be done almost at no cost if I create many accounts and keep paymng myself

i agree upvalue is more susceptible ot sybil attack, but thats not a problem yet... when it is the solution might be to have an automatic lock on the funds before they are sent... also mitigated by the treechat fee. other solutions might be a zero knowledge proof of humanity