No, that's not what the email from NSA would say. It would not say "there is a risk of your systems being compromised by cyberattack" in general terms, which is what IT already knows. It would say "your systems are vulnerable to these specific attacks", which IT does not know. So yes, getting this new information should change the risk-benefit calculation dramatically.
For example, a hospital IT department might get an email telling them that their MRI is exposing remote desktop to the internet with default credentials. They know that. They don't change it because if they do, their vendor will drop support. This is a real thing that real medical hardware has to deal with, and it's only slowly getting better.
A big industrial company might easily have it worse than a hospital. Fixing the specific CVE on a specific port on a specific machine might mean having to retire a whole series of obscure, niche bits of SCADA hardware that don't support anything modern. It's like all those IoT gadgets that don't support 5GHz, writ large.
https://en.wikipedia.org/wiki/SCADA#Security_issues
Somewhere between those two, you have your well-run Windows network. It's probably a month to several months of patching behind. IT has a whole process to test any new patches for stability and compatibility with line-of-business software to ensure that nothing breaks. Knowing that their systems are vulnerable to the CVE that's fixed by a patch they're testing - or tested and found broke something important - might not always help them very much.
Realistically, I find it not credible to believe that nobody in big infrastructure companies with IT departments is aware that they have vulnerable systems. I find it far more likely that people are aware and people in positions of leadership making decisions about risk have decided that these risks are acceptable.
Do you think getting an email from the NSA telling IT what they already know is going to change those calculations? My experience with bug bounty programs is that leaders who make risk decisions are more likely to shrug and say "I know, we're OK with that risk".
I realize that this is a personal judgment, and other people may have had wildly different experiences.