So explain to me how does the merit system prevent heroes and legendaries from posting equally low effort posts
Merit doesn't stop high-ranking spammers.
But there's this:
6704 permanent bans (including nukes):
4228 Newbie
1244 Brand new (nukes I'm assuming)
460 Member
328 Jr. Member
276 Full Member
99 Sr. Member
40 Hero Member
22 Copper Member
7 Legendary
87222 merits destroyed:
13 merits per ban on average
251 single-merit users banned
High ranking spammers will never earn back their rank thanks to the Merit system. And although the Merit system can't stop the existing spam problem, it stopped it from getting worse.
You (KingZee) seem to be on a tirade against the Merit system. You've been gone from the forum for a while, so you've probably missed that many others did that before you. It's not going to change the Merit system.
In an ideal situation we wouldn't need the Merit system, or even better: all posts would deserve Merit. But we're far from that, and Merit is the best we've got for now.