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.

I'm struggling not to be sarcastic here, as I'm not quite stoked about Canadian Tire owning most of what's left of Hudson's Bay Company. It's pretty undeniable proof that age and revenue will not make a company immortal or invulnerable though.

Even if the purchasing entity is backed by a foreign country?

Valuations are not permanent. Amazon dropped 90% during the dotcom bubble. And there is always another financial crisis coming.