The domain name of the holding company is not going to be Alphabet.com but the new gTLD abc.xyz.
“What is Alphabet? Alphabet is mostly a collection of companies. The largest of which, of course, is Google”
“Alphabet Inc. will replace Google Inc. as the publicly-traded entity and all shares of Google will automatically convert into the same number of shares of Alphabet, with all of the same rights. Google will become a wholly-owned subsidiary of Alphabet.
Our two classes of shares will continue to trade on Nasdaq as GOOGL and GOOG”
For the record Alphabet.com is owned by BMW but is not resolving but was active back in March of this year, so Google most certainly could have bought the domain name Alphabet.com but instead went with ABC.XYZ.
Here is a screenshot of Alphabet.com when it was resolving: