This is pretty much the classic line against any startup taking on an industry with established incumbents, and, as always, fails to account for the incumbents'

This is pretty much the classic line against any startup taking on an industry with established incumbents, and, as always, fails to account for the incumbents' complete lack of agility and the startup's long term vision. This isn't to say that Brex will be a huge success. But rather, that your argument is one against any startup entering an established market.