Ok, say Google sold their spreadsheet for installation on corporate servers. That would address the firewall problem. Would it be enough to kill Excel? What more would they have to do?
I doubt it - the spreadsheet in Google docs, even at a casual glance look to be missing some key features that are incredibly useful (Trace Precedents/Trace Dependents being one that jumps out at me).
There is also the fact that there are an awful lot of "mission critical" spreadsheets out there with huge amounts of VBA in them - I can't see Google supporting that (and you probably couldn't as a lot of these things call client side objects through COM).
To be honest, I don't think users want an Excel killer. Yes it has weaknesses (why it still can't handle Unicode CSV files properly is still a mystery) - but overall most people just see it as part of the standard corporate computing landscape.
You could perhaps make the whole model more interesting (e.g. by having better support for the hierarchical multi-dimensional data that finance people in particular just love) but then you'd probably lose the wonderful simplicity of the basic 2D sheet.
[NB I've spent a lot of time building finance apps that have Oracle Hyperion as a backend - which is a specialized 12 dimensional hierarchical database that is the market leader in its particular niche].
