Why do people ride a horse when you can buy a car?
Why do people drive classic cars, when you can buy new ones?
Why have antique furniture, when you can get new from a store?
I'm not an expert I just know what I like, and I'd an MDD case, that ran as fast as a brand new mac pro. Ran all my old programs, plus any any new one I loaded. Ran silently, reliably and was affordable to by budget. Has a mac OS9 environment, but that has all the capabilities of El Capitan.
That computer and OS do not exist, so I'll keep my G4 and leopard, for now at least. I got a G5 to replace it, and there it is, gathering dust since I bought it in January. Nearby the mac pro I got to replace the unused replacement, it too gathers dust. I will change over. One day. Probably. But not today, I think.