Old Garage Roof

Roof trusses may be used now.

Yes, I would trust a cable more than these 60 year old 20 foot long

The problem isn't the 2x4's; they've certainly got more than enough strength in tension. The weakest point is almost certainly the connection not the material.

The problem is as your first posting noted the effects of a leak over the years causing the joint to fail or perhaps the ends have also enough damage overall besides simply at the nailed (presuming) locations.

If you pull it back in shape and gusset it up back to solid material including a scab or two if needed to provide solid material and then fix the roof leaks it should last indefinitely.

If you're really ambitious, add an additional one between the existing. :)

Here's a picture that explains the differences between collar ties and rafter ties.

Is this what you have?

I assume you mean as a top (upper) chord not a roof rafter?

If I understand the OP question correctly, I think I agree with you. Too many people worry about the material not realizing often the connection is the problem or the weakest link.

Sounds like it is time to cut out the rotted materials and bolt the structural pieces back together with a proper connecting plate...

You are probably correct but I think it's safer to just start with all new stuff. I know it adds to the cost but I'm thinking of safety and longevity down the road. Of course that's the OP call.