Rentable defense is already a thing, but rapidly deployable mini-interceptors like Anduril and many others, or electronic countermeasures could plausibly become much more widespread.
I guess I am splitting hairs but "spare capacity" heavily implies it's a non physical resource or it's able to be used in an instant. Almost like how if you had a global based missile system like a GBI (or not quite global but long range like a THAAD) you could near instantly have someone "bid" to use your missiles in an emergency scenario. Building short range interceptors and selling them or renting them is closer to the model of AWS itself, building a knowledge base hosting your own platform (Amazon.com retail) and then selling that knowledge to others. In this case building anti missile systems to protect data centers and then selling a packaged model to other companies. But it's not "spare capacity", it's selling expertise and helping to fund your own R&D.
>electronic countermeasures could plausibly become much more widespread.
don't forget the amount of power available in the datacenter. You can easily redirect say just mere megawatts to electronic countermeasures (would shut everything around down) or microwave and laser weapons. That for example is just 60KWt https://www.youtube.com/watch?v=cvQV7Mt02q4
