How is this even a case? moreover the fact that MS is claiming it is for benefit of Users is itself suspicious
I'm reminded of those Safari-based jailbreak methods on iOS from the past.
How often do unsophisticated users identify where malware came from correctly?
I forget the name of it, but there's a fork one of the open source browsers that's distributed by neo nazi types. I stumbled onto it trying to find a copy of XP I could use in a VM to run some ancient software. I wouldn't at all be surprised if it had a backdoor.
Also when you have a company the size of MS talking about a small decision like this as if the whole company is onboard and actively engaged in the decision making process is naive. It was most likely a decision made by a few people at MS. Not saying that excuses it, but often individuals in companies have their own misaligned incentives and make certain decisions for their own benefits.
If the unsophisticated user performs a search with the OS, and the links get clickjacked by a cracked browser and the user ends up installing malware as a result, they're not going to blame Firefox, or the malware-writer. They're going to blame Microsoft, because they were just using the OS. What's more the vulnerability won't be something MS can fix because it will be the fault of 3rd party software. So to prevent that possibility MS want end-to-end control over the search results.
Having said that I think a lot of the recent bad will toward MS is self-inflicted. One easy thing they could do is allow the paid versions (Pro and up) of Windows to have an "expert" mode that lets the user configure their OS the way they want, including removing all the telemetry and making all unessential apps/tools entirely optional, including Edge of course.