Ballistic missiles do not cost only 100k USD to build. They are very unlikely to ever be that cheap. Rocketry requires enough precision to not explode on the launcher. Ballistic missiles with conventional munitions are only useful for point targets. Cluster munitions like Iran uses are an admission that they aren't targeting specific systems, aren't expecting to penetrate defenses, or other reasons why they would waste a ballistic missile on the modern equivalent of the Paris Gun.

Harassment weapons don't do much. None of the harassment campaigns done by the Nazis for example really amounted to anything.

Modern Shaheds can be possibly built at a scale to affect that, but we really haven't seen it happen yet. That would be something like thousands launched in a single wave against a single city or installation. But they still lack the precision and warhead to be targeted meaningfully.

You need WW2 industrial scale manufacturing lines worth of Shaheds to get beyond harassment. You need to be producing hundreds a day or more. That kind of industry is nearly impossible to protect from your adversary so unlikely to take shape.