It's not few. It's a very common mistake. Say I have 20 10G ports on line card. Mid size ISP has from hundreds to thousands routes. Thus we will end up with ten

It's not few. It's a very common mistake. Say I have 20 10G ports on line card. Mid size ISP has from hundreds to thousands routes. Thus we will end up with tens of hundreds ACLs per line card. That affects performance, dramatically. Say on older yet supported Cisco 7600 you can apply about 10k entries per port, ~ 100k totaling per card. That's limit, after that performance degrades. Another question what to do on peer-peer links, like say NTT<->Centrylink? Both on them have interconnects across the globe ad sending each other hundreds of thousands prefixes on 100G ports. That's the challenge as well. Hardware is not there, it's coming slowly but not yet there. Most of the big players DO one or another form of ingress filtering on some ports. Valid point is that smaller players are arrogant ob the issue and dump all all the garbage to the upstream.