RE: we don't seem to know how to build software that connects to public networks
I think we do, it's just that Microsoft's approach is too monolithic and complex. Unfortunately "nobody got fired for buying Microsoft products" trickles right down to companies with a handful of employees and they install Exchange Server to handle their email. They struggle to configure it and leave it at the stage where it limps along, keeping their fingers crossed nothing goes awry.
The following comment probably has these people in mind:-
"Microsoft, meanwhile, has issued additional mitigation advice for those unable to patch Exchange Server."
There is no mention in that mitigation of throwing in the towel and migrating to some kind of hosted solution, which in-house IT people now have the perfect excuse to recommend to their superiors.