You could theoretically determine both static and kinetic coefficients of friction by conducting a thought experiment along the following lines:

Slowly increase the incline angle until motion just begins. The tangent of that angle is roughly the coefficient of static friction. Once motion begins the friction force abruptly transitions from static to kinetic. For a diagram of this transition, see the "Friction Plot" in the following link: http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html#kin

Since the coefficient of kinetic friction is generally less than static friction, if there is no change in the angle, the friction force acting up the incline will now be less than the gravitational force acting down the incline for a net downward force and acceleration of the object.

Now imagine you are able to slowly lower the angle while monitoring the velocity of the object until the velocity becomes constant. At that lower angle the upward kinetic friction force exactly matches the force down the incline. The tangent of the lower angle would theoretically equal the coefficient of kinetic friction.

Hope this helps.

For example, if an object stays still at 15°, but moves at a constant velocity at 20°, the kinetic coefficient of friction will be greater than the static coefficient of friction. How does that make sense?

If the object is still at 15$^0$ all you know is the static friction force up the incline equals the force down the incline. The static friction force continues to match the downward force until the maximum possible static friction force is reached. The only way to find that out is to keep increasing the angle until motion just begins.

You could theoretically determine both static and kinetic coefficients of friction by conducting a thought experiment along the following lines:

Slowly increase the incline angle until motion just begins. The tangent of that angle is roughly the coefficient of static friction. Once motion begins the friction force abruptly transitions from static to kinetic. For a diagram of this transition, see the "Friction Plot" in the following link: http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html#kin

Since the coefficient of kinetic friction is generally less than static friction, if there is no change in the angle, the friction force acting up the incline will now be less than the gravitational force acting down the incline for a net downward force and acceleration of the object.

Now imagine you are able to slowly lower the angle while monitoring the velocity of the object until the velocity becomes constant. At that lower angle the upward kinetic friction force exactly matches the force down the incline. The tangent of the lower angle would theoretically equal the coefficient of kinetic friction.

Hope this helps.

You could theoretically determine both static and kinetic coefficients of friction by conducting a thought experiment along the following lines:

Slowly increase the incline angle until motion just begins. The tangent of that angle is roughly the coefficient of static friction. Once motion begins the friction force abruptly transitions from static to kinetic. Since the coefficient of kinetic friction is generally less than static friction, if there is no change in the angle, the friction force acting up the incline will now be less than the gravitational force acting down the incline for a net downward force and acceleration of the object.

Now imagine you are able to slowly lower the angle while monitoring the velocity of the object until the velocity becomes constant. At that lower angle the upward kinetic friction force exactly matches the force down the incline. The tangent of the lower angle would theoretically equal the coefficient of kinetic friction.

Hope this helps.

You could theoretically determine both static and kinetic coefficients of friction by conducting a thought experiment along the following lines:

Slowly increase the incline angle until motion just begins. The tangent of that angle is roughly the coefficient of static friction. Once motion begins the friction force abruptly transitions from static to kinetic. For a diagram of this transition, see the "Friction Plot" in the following link: http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html#kin

Since the coefficient of kinetic friction is generally less than static friction, if there is no change in the angle, the friction force acting up the incline will now be less than the gravitational force acting down the incline for a net downward force and acceleration of the object.

Now imagine you are able to slowly lower the angle while monitoring the velocity of the object until the velocity becomes constant. At that lower angle the upward kinetic friction force exactly matches the force down the incline. The tangent of the lower angle would theoretically equal the coefficient of kinetic friction.

Hope this helps.