Calculating Rs Value for 32kHz Pierce-Gate Crystal Oscillator Circuit
I didn’t see where you read it has to be such a large value, but this barely is enough to drive steady oscillation.  Your calculations are not correct.

The correct value ought to be taken Only from the XTAL OEM datasheet. Failing this, it is about 10k Ohms times the thickness of the Xtal in mm. E.g. 20k ~ 40k Ohm.

The reason no one tells anyone why this low uW power level has a limit is.   It is not from the resulting heat but rather the breakdown voltage in the XTAL which amplifies the internal “piezo-motional capacitance” to reach > 10kV x the applied input voltage.  

This is the
approximate Q of the resonator. (10k min)

When it breaks down then the power is absorbed immediately to damage to Xtal lattice structure interface and loss in performance.


If you operated without Rs and it still works , consider yourself lucky. ... Like not getting hit by lightning under a tree.
If it performs well with the proper Rs, consider this case closed.
if it still performs poorly, consider that proper layout must use short &lt;&lt;1cm connections, then any handling stress, that it may have experienced, including; 


ESD, excess solder time>3s, dropping 1m on a hard floor and operating without Rs.



EDIT July 20:
There is also a Low Pass Filter (LPF)function for harmonic content which contributes to the power dissipation but not the resonant frequency tuning.  This breakpoint occurs at Rs*C1 = Tau = 1/(2pi * f). 

The -3dB breakpoint occurs when Xc(fo)=Rs.  In this example C1=20pf fo= 32kHz thus Xc=(1/2pi f C)= 249kΩ 

The formula used initially was for the power rating where larger resonators have higher Q and thus higher minimum values of approx 10kΩ /mm thickness.  There is a significant margin before signal attenuation occurs at the fundamental at 250kΩ. if 2.5MΩ  was used then the inverter gain needs to compensate for the 20dB loss to achieve a square wave out with excess loop gain.  32kHz Xtals have a parabolic peak which is flattest and gives max f. at room temp then lowers on either side.