To be fair, they kinda already tried this. Early versions of Edge were touted as being Chromium but faster and more efficient. "Technical" people didn't buy it because of their mistrust of Microsoft, and normal users didn't really care about using less RAM.
Now they've added so many "features" even if it is more efficient or whatever it's annoying enough that technical users don't want it, or have to spend so long disabling things like coupon popups and some weird desktop search bar and the sidebar thing - that they just don't want to deal with it and go back to Chrome.