But in the specific case of launch padding, another microsat company could just as easily buy that same extra capacity from SpaceX. Sure, that's assuming SpaceX doesn't cram every launch to the max. There's also the opportunity to cross-shop a bit with other launch systems.

They'll need to predominantly do them on polar missions, in 2017 SpaceX had only 5 polar missions. Also, for missions that are already launching large numbers of satellites (like Iridium flights; 4 of the 5 polar missions in 2017 were for Iridium) this may be harder as it will require more fuel and a relight after an extended mission in its primary payload orbit.

EDIT: grammar fix

I don't think they're going to launch anywhere close to even 1/10 their network this way; there are too few scheduled launches and too many satellites in their planned network, even if they were able to overcome the technical obstacles you mention. Depending on the margins they earn, the few (if any) they launch this way may be a meaningful cost savings.