I believe the mistake is the same one that dates back to IE7/8 in that enterprise customers are at all given the option to always load intranet sites in "compatibility mode". It's really stupid that because of an Oracle-enforced GPO policy every time I try to open most of my company's intranet sites in Edge, GPO policy pops it into a new IE11 window, and corporate-enforced IE11 add-in pops it into a new Chrome window. This is madness, and there's no way for me locally on my machine to tell Windows that no I don't care about the GPO policy, I really just want to open it in Edge and skip the other two browsers entirely.
I do think a mistake was that IE11 was included at all in Windows 10 to appease corporations, instead of just ripping off that particular band-aid. But doubling down on it and allowing corporations to default to nothing but IE11 via group-policy is the biggest mistake. GPO footguns like that are why corporate America can't have nice things, and I'm increasingly of the opinion that it's time Microsoft started dropping footguns from GPO.
(Also, note that the root problem in this particular workplace remains Oracle and their web apps that are stuck in IE6 worst practices but cost millions of dollars each year to continue to operate. Oracle is the real villain here.)