It's popular to explain why they created an umbrella company and offer your theories. But I am convinced of this: the general public is not aware of the reason at this time.
I'll offer a very general reason they would do this, I don't have the details: doing this creates a financial/legal advantages for Larry Page and Sergey Brin. What kind of advantage? Why does it create the advantage? I don't know. But why do business people do anything?
I could be wrong, but I don't think it is for the "soft" reasons that people are positing now, i.e. it explains the structure better, it highlights that Google is not a "conventional company". This is a big change, and probably an expensive and time-consuming one. And making everyone aware of this "Alphabet" brand is a big pain in the ass.
I think the reasons they did this are very specific and concrete, but we just don't know what those reasons are at this point.