MOBA as a genre didn’t come from WC3. There were quite a lot of predecessors to DotA, both in WC3 itself and in first StarCraft, namely Aeon of Strife is believed to be the first popular MOBA custom map out there.
Blizzard didn’t decide that quirks of WC3 engine are dumb. Yes, they wanted to make a simpler MOBA, but the main reason for lack of funny stuff from WC3 is that they used Galaxy engine for the game, the same one StarCraft 2 was built upon.
And HotS feels less complex not because of Galaxy’s vs WC3’s quirks (the former has plenty, too), but because of lack of gold and shop, shared experience and an actual tutorial at the beginning of the game.