A very basic question about the concept of feedback in oscillators
Feedback in sinewave oscillators is actually stronger than the exact minimum to keep on the oscillation. Of course the amplitude of oscillations will grow, but in practice there's allways a limit how much they can grow. The amplifier starts to distort and this stops the growth. If one wants pure sinewave, he must filter out the distortion and leave the fundamental. In well designed LC oscillators the LC circuit does this often well enough for several purposes.

A RC circuit in the feedback route can well have enough steep phase shift vs frequency dependence to make the oscillation to happen in a stable enough frequency. But RC filters generally cannot kill the harmonics as efficiently as LC circuits. Low distortion sinewave is still possible if the signal chain has a control circuit, which measures the output power and reduces the feedback or amplifier gain as soon as the output power has reached the wanted value. This is known as automatic gain control (AGC). The amplifier amplifies a little more than is needed for oscillation and the AGC circuit reduces the gain or feedback factor proportionally to the output power. 

Very simple AGC solutions (often not more than making the amp clip softly) can produce a sinewave which seems perfect in oscilloscope. This is not enough for distortion measurements of high quality amplifiers, much cleaner sinewave is needed. To get it one needs a well designed control system to do the AGC. (+low distortion amp in the oscillator, too).

Here's an example of RC sinewave oscillator with AGC circuit. AGC circuit has so called PI controller which searches the exact needed gain.