Would be too broken as there would be no way for the enemies to play around this. Atleast as an enemy Dynamo, you can counterplay magician by simply being out of his sight which wouldn't allow him to steal it from you.
In general, the community is already divided about Dynamo buying refresher for double ults. Unlike Refresh which has a cast time now, this hypothetical change can guarantee an instant inescapable double blackhole which I'm pretty sure the community would be furiously against.
But then again, sometimes the devs shouldn't care about the community's opinion since the community will always find a way to cry about something. In my own opinion this could work but only as an aghanim scepter effect or something like that.