As I recall, they specifically implemented it in order to facilitate Outlook’s web interface – their collective regret may be tempered by the satisfaction they got from all those high-margin site license sales to which that feature contributed.
I wouldn't mind hearing a snippet of that discussion. Why would MS regret XMLHttpRequest?
Microsoft twiddled its thumbs while the web grew up and around Windows using the technology the company introduced.
That makes complete sense. Never thought of it that way.