Even so, SpaceX can launch 50 satellites in one launch, and each suborbital interceptor can only hit one. And the "if it had navigation" part is doing a lot of work in that sentence; a satellite interceptor kill vehicle is not easy to build. Are 50 of them cheaper than one Falcon 9 launch with a 15x reused booster? I think "maybe" is the only possible answer, but probably the cost of killing all the satellites that way would be around the same order of magnitude as putting them up in the first place. You would need to build multiple whole factories just to make the thousands of interceptors you would need.

I wonder how hard it is to take out multiple starlink satellites right after launch, when they're all on essentially the same orbit?

It's an interesting idea. They are deployed and start spreading out within minutes of launch. You might be able to take them all out in the first couple of orbits. I expect it would be much harder after that. SpaceX could possibly defend against this by varying the orbit so you don't know where to place your interceptors, and maybe by having the satellites boost apart from each other sooner.

But you'd also need to take out most of the on-orbit satellites before it would be useful to blow up the replacements.

Seems like it’d be easier/more effective to take out the launch vessel.

Sure, but you'd have to be relatively close to the launch pad. The advantage of doing it in orbit is you can do it from the other side of the world.