doubt on application of stability criteria
If we assume G(s) is strictly positive thus:
$$
\frac{KG(s)}{1+KG(s)}
$$
$$H(s) = 1$$
is a closed loop negative feedback system where it is assumed that H(s) which is the feedback component which is equal to 1.

We cannot substitute the plus sign for a minus sign which would make it a positive feedback system and therefore make it unstable.

$$
\frac{KG(s)}{1+K H(s)G(s)}
$$
$$ H(s)\neq 1 $$

For a system to be BIBO stable it must satisfy the magnitude and angle criteria. Which relates to the poles in the s-plane of the closed loop system.

Basically if you look a Root Locus diagram of a system all the poles should be in the left half plane and the phase plot should be within \$ \pm\pi\$ but thats not strictly true. A system can have range for which K the system is stable.