Stalin wasn’t really that kind, he was very cautious about invading other polities unless he could be absolutely sure of no real repercussions.
If the Nazis hadn’t been a thing the butterflies for Romania are vast. Critically, the Romanian government before Antonescu was very Francophile and had a good relationship with France and other countries. If Hitler hadn’t agreed with Stalin to carve off their territory and the fall of France hadn’t badly undermined the original government’s position they would still be in control. In that case, there would be very definite repercussions for invading Romania. That would keep Stalin at bay.