Magnitude of Tension
If you stretch a string 1970 style string-theory string by about 10−15m then you add enough energy to produce two quarks, or about one Dalton which weighs about 10−24 grams.
The force is (10−24 g) c2/10−15m
= (10−24 g) 1017(m/sec)2/10−15m
= 108g m/sec2 = 105 Newtons of force.
I don’t know why some thought that there might be primordial strings left over from the beginning if there was such an easy decay mode.
Perhaps the “three quarks per lepton” rule precludes such decay outside the nucleus.
(But what about the meson?)