I see what you're saying regarding the original paint, but there is quite a substantial amount of bare metal on the non-drive side chainstay showing from heel rub and more underneath the BB shell.
I could try to find a colour match and touch it up with some paint, but it'd probably be safer in the long run to strip it all back, check for any hidden problems and then have it re-coated.
That way it will last at least another 20 years or so.
I'm riding it in the meantime though.
I think the important thing is to Kurust all exposed metal - that will stabilise it. Bare metal isn't really a problem, though, it's bubbling that you need to be wary of because that means rust coming from the inside. If you have that, I would totally agree with you.
I don't think an aftermarket powdercoat job is safe for anything like as long as 20 years though. The trouble is the steel usually isn't primed, all the paint goes on as one layer, which is very hard and stiff. So if there's any crack in the paint over the years, moisture can get in and get between the frame and the paint, causing unseen problems that can get bad before they're apparent.
If the basic paint is sound, my guess is that repairing it will last at least as long as you should leave a powdercoat before re-coating.