In my view there are some incentive also for giving merits, if anyone gives merits to low-quality post then he will be punished by losing some of his existing merits points and if he give merits to high-quality post that is helpful for people then he must get a reward for this as well.
Punishing members giving merits to low quality postsThis is already happening.A lot of members who sent merits in bulk like 20 merits or above for a single post were easily recognized, reported by members here and then given negative trust by senior members. Unfortunately, members are still not stopping this practice as evident from such cases being reported frequently even after the merit system is quite old now.
Rewarding members giving merit to high quality postsWhat type of rewards are you expecting ? Monetary rewards or merits ?
This suggestion is not practical in my opinion as most of the merits are being given by merit sources who are already established members.