Old neck injuries limit my neck mobility so I've used mirrors since I resumed riding in 2015. I don't race anymore so it's no big deal.

I've noticed several cycling acquaintances who used to race and ride without mirrors have added mirrors as they got older. No particular reason given but I suspect it's the same old thing -- with age everything gets a bit stiffer, mobility suffers and our reflexes and hearing suffer a bit. I don't like to rely on listening for approaching vehicles, although my hearing is still pretty good. But some vehicles now are incredibly quiet, especially with smoother tires on smooth asphalt.

Even one of the fastest guys I know locally, who holds many KOMs, uses a pricey Italian made glass bar-end mirror on his road bike. Another friend used to razz him a bit about it. Now that guy has the same mirror on his road bikes.

I use the similarly design but much more affordable Sprintech mirrors linked above on two of my drop bar road bikes. However my hybrid with swept bars (Nitto Albatross) have bar-end shifters so that space isn't available for the Sprintech and similar bar-end mounted mirrors. I mostly got those so I'd have a mirror on occasions when I wear my aero helmet, with no way to mount a helmet mirror as I do on my main helmets.

I also use Take-A-Look mirrors on my two main helmets.

I considered an adapter for my Mirrycle handlebar mirrors, but discovered I could get by with just the helmet mirror, so that bike still has no mirror on the handlebar after two years. I keep thinking about trying a clamp mount and adding a mirror, but I guess I'm satisfied with it as-is.

I've seen a few folks add clamps to the Mirrycle and similar design mirrors, that put the mirror below the handlebar, rather than above. That seems to suit folks who prefer lower bars, or flipped North Road bars (path racer style).