All ISPs offer static IPs, of course.
I suspect most people won't want to mess around getting Static IPs sorted and will just go somewhere else to mine.
If that restriction was removed, then they might use your pool.
Cloudflare is not cure-all DDoS, for example moneropool.com use it, but they have been attacked recently. If user have a dynamic IP we can add a range of IPs that use ISP.