Satellite self-defense systems are actually pretty efficient at protecting a satellite, and they don't pose an undo risk to the rest of us in the ground.
But any kind of orbital missile defense system, is going to have to be inconceivably large and prohibitively expensive, just to reach a point of basic viability. The planet is huge. And considering that the kinds of attacks that systems like these are designed to prevent, will be traveling at orbital velocities...you need to have missile systems everywhere. If you don't, then you won't be able to "catch" the hostile missiles you're trying to intercept.
That means potentially thousands, if not tens or even hundreds of thousands, of launch satellites in orbit. And we're not talking about small satellites here. In order to accelerate to the kinds of speeds required to intercept an ICBM, these missiles will need a lot of fuel...or they'll have to have even more of them in orbit, so that any path that ICBM takes will have at least one defense satellite in a position to passively intercept it.
Low to medium Earth orbit would have to be blanketed in these satellites. And that's just to protect one country. If the US starts doing this...so will China. And Russia. And everyone else who's afraid of these satellites will be used offensively as well as defensively. Because the same missiles that can potentially intercept an ICBM in flight, can also do an incredible amount of damage to a surface target, with little to no warning. They would be everywhere above us. All of us. And could just drop a missile directly down on any point below them.
This gets even more complicated, when you think about the satellite to satellite defenses required to defend each one of these launch platforms from each other. Logistically speaking, it turns into a cluster-fuck, really fast. You could potentially see billion dollar satellites getting shot out of the sky as soon as they reach orbit, and before they have a chance to reach their intended firing positions. How many times could you stand to lose them, before the cost of trying to send them up there became prohibitive? How much of your GDP would you be willing to spend in order to simply maintain that system?
