Well, ask a software dev about the costs of supporting and maintaining a legacy codebase that connects to many outside resources that are themselves everchanging. And then think about customer support costs.
When I speak to managers/entrepreneurs who are looking to enter or exit the brokerage business -- futures and/or equities -- I hear a surprising amount of emphasis on AUM. You might think, what does that matter, since rates are zero, there's no skim for the broker on that side of the business. But they all think ZIRP is temporary, and they want to build assets now with the expectation of making money when rates go back to "normal." They're all aware there's no money on the commish side, since IB and similar firms have bottomed out rates on the retail side -- let alone the near-zero rates on the institutional side. And the costs of running a brokerage -- compliance, paperwork & etc -- are rising because of Dodd-Frank and other bright ideas out of Washington... The point is, AUM is in demand now, more so than turnover. If you're a seven-figure account you'll get princely treatment from TD, or any of its competitors, and they'll take seriously your needs and concerns. That's the reality right now.