Would having back up servers which kick in instantly when the primary server goes down kill a DDoS attack or just delay it? How many networks, distributed, would a midsize company need to withstand a DDoS attack?
Maybe. It depends on whether or not you have a single point of failure that is the target of the DDoS.
If you have a domain, an IP, an ISP, and a server farm all serving your business, and this gets attacked, then, standing up a new domain, IP, ISP, and server farm will help to mitigate the attack, as long as the attack does not change to target your new location.
If you do not change all those things, like the domain, then the attack could automatically switch over to the new IP, ISP, and server farm once you redirect the domain to the new location. Same thing for each element.
So, yes, given the idea that a DDoS attack is limited in scope, then yes, setting up resources outside that scope could work (until the attack retargets). But, any single point of failure could bring your mitigations down.