Now, I guess, if an ISP ignores a DMCA takedown request, all that happens is that they too can be sued for copyright infringement of the content.

Now, I guess an ISP _could_ individually evaluate each DCMA takedown request, decide that some of them are completely illegitimate and there was in fact no infringing material, and thus that they have little legal risk to leaving it up, and probably nobody's going to take them to court over it, and if someone does, the ISP will probably still win and they're willing to deal with it.

I think it's pretty obvious that it's not really reasonable to expect a business to do this (by assigning an actual lawyer to evaluate every request? By trusting less trained/certified people to make the determination? Even paying them minimum wage it's probably not affordable), although it would be pretty exciting if one decided to anyway.