It sounds like it's 25% better than what you get in your country?
If we're laying off 20% of the company, I'd expect the package to be somewhere in the 6-12 months range. To me, that would seem more fair, considering they are completely restructuring the whole company.
In the US luckily the job market is still quite dynamic (and extremely tight by historical standards) so only in an extreme scenario would someone with Dropbox on their resume need 12 months to find another job.
They might have to take a pay cut from a 99th percentile salary to a 96th percentile salary though. Rough, I know.