The Elves did not invent the Ents, they merely 'awakened' them.
I think that it is plausible that whenever an Ent got bored with just standing about for thousands of years he became 'tree-ish' (who could blame him?) Trees have life and die, so the 'tree-ish' Ents would wither away likewise.
By the way, excellent point about the hearts
Imladris.