What would you even value Coinbase at? If I was Armstrong, its not the current valuation that would interest me, its the future valuation I would price my product on, and what is that value?
If bitcoin goes to $100,000 and I believe it can, then the total market cap is going to be something like 4-5 trillion dollars in my opinion, and if Coinbase has a strangle hold on the entry point into that market place, which in my opinion they do, that is worth way more than current valuations of Coinbase could ever suggest.

I suspect Coinbase would heavily regret accepting any offer of buy out, HEAVILY.