underwriting and M/A and whatnot is a whole lot different than just managing deposits and loaning money
The models will change, as will the shear size, simply because a big profit center will no longer be in their purview.
IBs are going to be forced under the regulatory umbrella now that their actions have compelled taxpayers to bail out the credit markets. The highly leveraged IB shadow banking system is where much of the damage to the credit markets is occurring. Maybe if we return to some version of Glass-Steagall then the IBs would be able to survive on their own. If not they surely will be tied down to a commercial bank where their activities will be far more restricted than they are today.