I would say they haven’t analyzed the risks properly, especially since we have both on prem and cloud proxy services.
But, it’s a boneheaded move regardless to try to migrate 93000 or so employees in 1 day. Some sites have bonded T1’s as their primary ISP link (4.5 Mbps), or even 4G/LTE. Even 3 employees with 3-4 Gb Outlook *.pst files would make that unworkable.
We’re MOVING to SDWAN which would help address the issue you point out, but with hundreds of sites (if not a thousand+), that’s going to be a multi-year project, of course.
Curious to know who's solution you're using for SDWAN? We're a SilverPeak shop. Their partnership with Microsoft is outstanding. We've received excellent support from both.