I think vanguard is perfect for the type of investor they attract. I probably log in to my vanguard 2 or 3 times a year. It is set to autodraft from my bank account and buy index funds every month and reinvest the dividends. So it is set it and forget it.
A slick interface and UI team that is trying to figure out ways to increase user engagement is just asking for your users to get in there and gamble away their retirement savings.