This is why we want Bitcoin nodes running in as many ISPs as humanly possible. Ideally on every ISP (although one wouldn't make it such a hard requirement that the system is unpractical).
In the end in a system with so many different constraints and where some of them are hard to measure or even agree to a target on, judgement calls need to be made.
I live in a rural valley supported by only one ISP. They have 10 GHz connectivity to their upstream provider. The DDoS attack completely saturated this connection. Presumably with better software in the upstream provider's router the attack could have been confined and it would only have taken out my personal Internet service. (I have had conversations with the head of my ISP's tech support, who positively confirmed what happened.) I know who their provider is, but I don't know the make and model of the carrier grade router at the end of this fiber optic link. So I haven't investigated further to see what could be done. Personally, I believe the lack of suitable defense is my ISP's upstream provider's fault, but for all I know their router vendor might not supply the necessary features, which would be trivial to implement, at least for a single attacked IP address. Unfortunately, because of the sorry state of telecommunications monopoly in the US, my ISP's upstream provider is a very large telecommunications company and if they are not providing proper DDoS support to their smaller ISP customers, your good suggestion as to having many ISPs supporting full nodes would not necessarily help.
If you have any suggestions as to a future conversation I might have with my ISP, I would definitely appreciate your suggestions. The people involved were overwhelmed by the magnitude of the DDoS attack that my XT node brought on. They had seen large DDoS attacks, but none of a size sufficient to totally overwhelm their network capacity before the attack on my bitcoin node.
If by some unlikely chance, you know of some way to track down and identify the miscreants, then that would be interesting too, but I'm not really sure I want to know the details, because they might provoke me to do something really stupid in retaliation.