When I lost the original rearview mirror I am convinced I looked absolutely everywhere in the car to find it. But recently, whilst helping someone strap into the passenger seat I saw it had slipped down into the small gap between the seat and the bodywork.
So I put it back on the windscreen, intending to remove the new extending one.
But when I took it for a ride I actually found it was quite useful. The original one is very solid and does not vibrate but has the drawback that it is very wide-angle. So you only see another car when they are right behind you. The other mirror is has a narrow field-of-view so you can see individual cars from some distance away.
So it makes sense to keep both, although I keep having to explain the reason to interested onloookers.