They could. They could even just give an AoE dot to replace single-target so there aren't extra buttons if that is a genuine concern. It would be less stress because during trash pulls less experienced/less confident healers don't have to tab-dot, tab-dot, tab-dot, tab-dot, tab-dot... (Personally I'm fine with a single target and an AoE target dot on the same bar, just thinking of the complexity concern being cited by SE.)
Add that to the recommended variety for one-button spammables for something that at least has increased utility.