I fail to see how that sort of things would have made the villains any less cartoonish or "villains of the week". That's still what villains in superheroes stories will do - kill the secondary tier helpers of the hero, or turn his friends against him.

IMO, that impression comes not at all from their actions or the relative lack of success of their plots, it comes from the fact Jordan gave his villains clichés motivations, often one-dimensional. This one is envious and his envy determines everything he does and that one is a scorned woman and that's all that defines her and son on. There's not a single one in the group who sees beyond personal interests (beside Moridin, who is just a human version of the DO who also seeks nothing but complete desctruction of everything). Jordan tried too hard to make them the poster boys and girls of specific personality flaws and it's virtually all they are. They're too much a freak show of dysfunctionalities to be realistic as a group of villains. Grey villains also have their flaws. They're more realistic, but readers tend to keep hoping they'll redeem themselves and disbelieve they could commit the evils they do.

Otherwise the series is large enough as it is without Jordan padding the main cast with expandable main players who, by definition, would need a lot of screen time to be "main players" before they are killed off. Most people don't enjoy reading hundred of pages of character development only to see it end abruptly. Sure, Jordan could have used an expandable main player to be killed by the mid-series and raise the stakes for all the others (he seemed to have seen Moiraine that way, but if so it didn't work so well as he foreshadowed her return too much), but if he did that too much in a character driven story, the readers who tend to identify a lot with the characters would get annoyed - and if you want the reader to follow you into a series so long, you need to be very careful about that. How would the people who love Nynaeve or Mat would feel if Jordan had killed them off in book 6. Martin, who has a more plot driven story despite having more realistic characters, is a good example of that: he's killed so many players and turned their whole build-ups in useless red-herrings that each time the shock value goes down and more and more you expect your favourite characters to suddenly bite the bullet. With Martin you have to play along and enjoy the story as it unfolds, it's useless to have hopes for the ending or try to guess what is and isn't relevant. Some like that, others don't. The British show Spooks has overdone it the same way - the surprise has become for a main player to make it to the end of the season!

In a story like Jordan's relying so much on the tropes of the hero's journey, killing Moiraine or Lan early would only have meant another character like her would have surfaced later (which it sort of did, with Cadsuane).