Value of loop gain for steady state oscillations
Non linearities in the transistor will inevitable make the loop gain equal to 1 at some peak to peak amplitude and, the natural impact of this is that the amplitude is difficult to precisely predict (not normally a big deal). The second impact is that the sine wave output will be naturally flattened on one or both peaks and, again, this is often not regarded as a big problem.

So, in short, the sine wave amplitude builds (because loop gain is greater than 1) and, as clipping starts to impact the sine wave shape, loop gain falls to exactly unity.