It might be too late now, but I've successfully negotiated (before signing) retention deals like this to be pro-rated in the event of non-voluntary termination. It's perfectly reasonable for exactly this reason, and companies have no legitimate reason to deny it.
Agreed, this is/should always be the deal basically - "here's a bonus so you stay while we transition" is the only reason many folks stay post aquisition. It should not be revokable, especially last minute.
To me its a massive red flag that Google would promise a cash bonus 2 years in the future, they could have given stock with an appropriate vesting schedule for retention. All the retention deals I have seen were done like this, only for the very short term (less than 6 months) a cash bonus makes sense.