At some point 900,000 routers with 100Mbps fiber might be a realistic user base that would be tremendous amount of traffic to smack people and that's without considering amplification attacks and such.
Thats assuming a volumetric attack, even just "request foo.co.uk every half second" would be catastrophic, 1.8 million requests per second would be a bit of a bugger to handle.
I guess ISP specific attacks (with ISP specific boxes) wouldn't be that much of an issue as the ISP could be blocked. Will deny service to all users there, but the fault is clearly with the ISP, so they have to fix it. It's much more problematic if a generic router that's being used across the globe has a vulnerability. Filtering traffic will be much harder and ISPs will deny responsibility as it's not due to their machines.
https://forum.ovh.es/showthread.php?14451-Informaci%F3n-rela... (use Google Translate)
As those connections spread (and they will - if you have optic fiber coverage, the slowest you can get is symmetric 50mbps), things will only get funnier
In any case I can make dozens of requests per second with only 1M upload. I'd say the amount of requests/second will only be limited by the CPU of the router.