There are similar ideas, like the Iron Curtain and Bullet Curtain, which both use a lot of tiny missiles to interfere with an incoming missile.

My idea is a mile high curtain, like a fishing net, woven from carbon fibre, as thin as possible.

It lies folded on the ground (maybe there are rows of them, miles apart…) and when activated it is sent skyward by rockets on each end of it. The rockets are angled slightly away from the curtain so that when the curtain is drawn tight horizontally, the rockets self-straighten.

Balloons are released when it reaches full height to keep the curtain up for a while. Afterwards, balloons and rockets are refreshed and the curtain can be re-used.

The netting of the curtain simply needs to be tight enough to probably stop whatever is incoming.

Conceivably it would be cost-effective relative to other systems, and a good part of required redundancy.