i definitely wouldnt jump to doomsday thoughts since my c1500 shift cable was laying under the carpet and just daily use by my dad made the shifter get wonky and eventually he broke the BTSI trying to throw it in gear. Loosey goosy. The sheath gets worn, then the strands get rubbed away, and then as soon as it can "kink" where the wear is when using it(usually near a loop) or the cable braids shave away, then the cable starts pulling itself apart lengthwise.
its a fairly common need to replace in these trucks, I suspect the sharp loops the cable has to take + the normal use of a truck are why but thats just theory. ESPECIALLY since it occured during a manual downshift. " In between gears" become possible because of the linkage not being in sync with the column detents.
If you hold the shifter lever AWAY from you when using it, you can actually find the exact detents since it will "climb" almost like a ladder (youd understand if you ever do it lol, hard to describe)