AOL used to be a US$ 200B public company, it was acquired by 4.4B.

Sun, Lucent, Yahoo all had massive valuations at their peak but eventually dwindled and got acquired.

It's always possible for a massively valued company to stumble, fall, and become a husk of what it once was. I don't think Google/Alphabet is immune to this even though their absurd cash cow from ads make it very unlikely at this exact moment.