I dunno about defense as a service since those are pretty short range systems you mentioned (how would someone go "buy" excess capacity), but datacenters already cluster around common resources (water, etc.) so group buying some equipment to put in a ring around the datacenter area seems like it would be what they do.

Yeah the use consumer grade rocket components made SpaceX become viable compared to bloated rocket companies. Short range anti missile systems are not large ordinance, they rely a lot on technology for tracking targeting, and they are not a "weapon" (as in they prevent damage not cause it except inadvertently) so it actually seems like something pretty feasible for a tech company. Build it with consumer grade hardware and you could deploy a ton of them.