They can launch a Pegasus just fine from an L1011 already. I'm sure that if that needs to be replaced at some point, another retired airliner conversion will be much cheaper.
Perhaps the stratolaunch aircraft will end up like the An-225, once intended for space hardware, but ending up as a for-hire oversize load freighter.
Except the AN-225 has an interior cargo compartment, unlike Stratolaunch. A dedicated cargo infrastructure would need to be developed for it. Not seeing that as a big business model.