Frame rust- common?

My 1982 LTD 1000 developed a crack around the kickstand. When tig welding it back, the metal was disintegrating. I suspect that water has gotten into the security cable port (no cable in mine, nor plug) over the past 35 or so years and weakened the frame. Any similar experiences? Should I start planning to replace? My welder friend got the crack back to solid metal, but is it just going to happen again?

It's a common problem on frames with the cable holder.
First sign of trouble is usually the bike leaning over more than it should as the mount twists away from the rail.
Usual repair is to sleeve and re-weld the rotted lower rail and re-attach the side stand mount and blank off the cable hole to prevent future water ingress.
Any good welder will be able to make a permanent repair and there should be no reason to replace the frame.

Yes in the UK the frames can rust, also from the inside out. Also in box sections like rear shock top mounts. Surface pinholes can be a lot worse when cleaned up to bare metal.

Your problem is more related to localised metal fatigue I suspect., and the side stand tab is an (occassional) known fault with these.