Light rail to DIA would not work for a host of reasons. For one, light rail can't share the same corridor as freight trains, which is why the B Line is also heavier commuter rail. Light rail to DIA would also be painfully slow and would not accommodate multiple passengers with luggage very well. And even if you didn't have to switch from light rail to heavy rail, trains were never going to run continuously from individual suburbs to DIA - the ridership wouldn't support that. That's why RTD shouldn't have cut SkyRide services that served areas that are distant from downtown Denver. Our SkyRide stop was eliminated, despite numerous complaints being registered with RTD and with our dimwitted rep on the RTD board. 

The transfer at Union Station is a bit of a pain, but I don't think that's the main obstacle that stops people from using RTD to get from the suburbs to DIA. Even if you could step right off one train and onto another, the trip would be too long for most people's taste.