As far as I know hypersonic missiles (as Russia has recently demonstrated) which are launched from high flying aircraft and go into low Earth orbit and then come down almost vertically on their target would be far more effective and cost efficient and less vulnerable to attack than any spaced based missile launchers which can't just sit over their targets ready to be launched at a moments notice.
Unless they're in geostationary orbit which would mean they'd be very far away and would take much longer to reach their targets than would hypersonic missiles.
Considering the weight of a fully loaded missile launching platform and the size and cost of the rocket needed to get it into high orbit, plus the fact that it would have to sit there, effective motionless making it a very easy enemy target.