It is well known that Opti bootloader can be burned onto a Nano, simply by treating as Uno.
I do this with most of mine.
So what are the reasons that there is a Nano specific bootloader ?
There isn’t? Nano predates uno, and has the same bootloader as duemilanove.
OK. I see I could have formulated my question more precisely, but I get the general idea that it is a historical matter.
So there are then only advantages in dropping the original Nano/2009 bootloader and replacing it with the Opti bootloader ?
Correct. If the Arduino team had done it when the Uno first came out (which was a bit before the flood of Chinese Nano Derivatives), we'd be in a lot less pain now WRT the whole "Nano (Old Bootloader)" thing.