In my experience, what ever that's worth,
over time, the strap is simply gonna fail...
On every old bag I've seen, the strap had failed,
often repeatedly....
If it fails along a line of awl holes incorporated into the top flap seam,
the repair is easy...just carefully open the seam there, usually only about 5 or 6
stitches... remove the broken bit from the seam...reinsert the strap...
awl new holes through the strap using the original holes as a guide,
and sew it shut again...done.
If it fails by stressing and tearing the back panel, well then the repair is
not so easy...
I have seen a whole bunch of repair attempts to correct
straps that tore off the back panel, and although many
attempts were very creative, they could never really
fix all of the damage done...
I have to believe the multitude of old bag makers that
repeatedly and purposely made that "perforation" or zipper
line did it for a good reason...
that reason being that they knew eventually the strap would need repairing,
and repairing or even replacing a strap that tears along such a line takes about
5 minutes and you never even know it was mended...
just my two cents worth..
tc