It's important to remember that these service aren't for the sole benefit of UK outbound traffic, and that Birmingham / the Midlands and Cardiff / South Wales are destinations in their own right for business and leisure passengers. Stansted is 2 hours drive, parking is diabolical - almost nearer to Cambridge than Stansted! Cardiff, given the useless road links, and pretty inadequate rail infrastructure makes it unappealing for Midlands destined passengers.
BHX will stand or fall on it's own merits, and frankly I'd expect Stansted to struggle with a 3-class offering, since most inbound passengers think of LHR and LGW as the main London gateways. Could see it doing OK with 2 class, but we'll see.