Surely it's not a requirement just like it probably isn't entirely necessary to replace the prop that struck the stake if you inspected it and like the other 3, no visible damage. The way I think about is the lifetime of the drone which for me usually involves one complete change of props. Personally it makes no sense to me to end up with 2 or 3 unused props when my drone goes out of service. I will say if you change all 4 to brand new props and you throw away the damaged prop, you should *keep* the 3 used but still ok props for use another [emergency] day.
Same with tires of the car (but that's not exactly apples to apples since wear is more apparent). If I blow one tire, unless the other tire is practically new, I would change two tires (not 4). Again, if the other tire is not too bad, perhaps it makes a good spare for the 2nd blowout and you're effectively back to square on.
My comment on replacement mostly around the extra props already given to you by DJI. There's a reason why they give you 4 spares, not 2 (although there may be some packages where this is done which means you can replace one at a time depending on the type). But this is likely only meant to get you temporarily back up in the air for the day. Agreed if you have to spend, spend, spend then you should take that into account but honestly, I don't think anyone sells props by the 1's which would be the chief complaint from those who determine it's best for them to replace only non-functional props. But, maybe that's to avoid confusing on actually buying the right type so min 2 makes sense. But yeah, don't go out and buy props just to replace props necessarily.
If you need
Mavic 3 props, I have
Mavic 3 props. I use
MA props for the colors and honestly, will never go back to OEM
Mavic 3 props and I bought the combo so I have a bunch. If I break a prop, my
Mavic 3 gets a
MA replacement. I will keep one OEM set for Care Refresh but when I sell the
Mavic 3 (which I will when Mavic 4 or 5) comes out, I will keep the extra OEM props since, in my opinion, including a bunch of extra props in your private sale does little or nothing to get a better price.