Arecibo was our best tool for tracking near-Earth asteroids... which we need to do.

Arecibo was not just a telescope, but a radar dish. With radar we could accurately measure the distances to solar system objects, and get substantial information about their surfaces. Without those measurements, we cannot plot asteroidal orbits accurately enough to ascertain when there's a threat of collision.

We need to build a new radar facility. But this time, instead of a single giant dish we can make a hundred normal-sized ones working in tandem.