"If Zamasu had not merged with the universe, why would Zen-Oh have erased it?"
Good question, but there is one bit you kind of left out. Zen-Oh did not swoop in and just erase Universe 7 before leaving. No, he erased the entire timeline.
Zen-Oh uses the word "world" when describing his target, and the word "world" is typically reserved for entire timelines, as they referred to as "parallel worlds" in
Dragon Ball. Had he have only targeted that specific universe, which would have singled out Zamasu as the sole target and added credence to the idea that he became the entire universe, he would have used the word "universe" which is what everyone in the entire series uses when referencing... universes.
Zen-Oh was already annoyed by Zamasu (see left) and on top of that, Goku -- someone he does not know because this is the future timeline -- randomly pulled him away from whatever activity he was doing to take care of Zamasu. It has already been established that Zen-Oh has a very short temper; in the past, he erased six universes because he was in an "unpleasant mood", and Whis has stated before that he could and would end up erasing
everything from existence if he ever got sour. Him erasing the entire future Multiverse because he was ticked off by Zamasu is just par for the course with Zen-Oh.