They explained the change to heat sinks lore wise as using heat sinks allowed a soldier to fire more rounds down range in a shorter amount of time compared to an advanded heat dissapation system. The trade off being you had to carry around heat sinks. Chances are they never implemented the ability to just wait for it to cool after running out of heat sinks was down to simplify gameplay. In the books and other media, there are several references to people doing this or using older weapons (M-7 Lancer).