If mods could IP ban people then redditors would know, and if they knew they had a shared IP then they might volunteer this, yes.

"Might and maybes" are again, sacrificing the good for the perfect. "Maybe this could happen, so we shouldn't have the feature" is an, ironically, short-sighted take on the problem.

At first glance it looks like you're trying to consider all the variables but you are (not you specifically, just those who might be against the feature) not considering the even bigger picture; the (potential) quality decline of reddit. Particularly its comment section. It's quite apparent that reddit's large communities are becoming, and can be, quite toxic. I don't think anyone disputes that. Being able to really block people from your subreddits would be a huge benefit for those communities which are plagued by some of the worst trouble makers and trolls on the internet (especially large subreddits, controversial subs, subs which cater to an immature/young demographic, preventing/stopping 4chan raids, etc...).

As reddit gets larger and larger, these tools become even more needed. Some subreddits are desperate for them. At this point, I think it's a miracle reddit has maintained the quality level it has. Unfortunately, as reddit grows and grows, even the best moderated subreddits can only hold onto that for so long with their mod's hands tied behind their backs & unable to remove undesirable elements from their communities.

But to answer your "maybe"; As I said, there are numerous, perhaps infinite ways to solve that problem. It's an easy problem to solve since it's a technical issue. One way right off the top of my head would be that an IP blocked user can no longer PM the mods of that subreddit. Isn't that kind of the point anyways? Then from there, banned users can use the new 'escalation page' where those who might have been unfairly blocked or got caught up in part of a large accidental subnet ban can fill out a form to send to the admins who then would do what they're already doing right now - look into it and take the appropriate action. Problem solved.

so I doubt they'll ever give mods the ability to IP ban.

An IP ban doesn't have to give actual IP addresses out. Again, it can be implemented in a way that gives out as much information as a regular ban does. Along with fixing modmail, the ability to IP ban (truly block a problem user) is the most requested feature by mods. I'd be surprised if we don't get it, or something very similar to deal with those sorts of things.