Input and output impedance in RC circuits
We can't achieve the 'ideal'. But if Zin is 10x Zout of the previous stage then the loading, attenuation, and interaction between stages will be minimal.


  If the frequency is set to too low, Xc goes to infinity and Zout
  doesn't become R


Zout becomes R plus the source impedance. Provided we stick to the 1:10 rule (Rb = 10 x Ra) we are OK. The only problem is that each stage multiplies the impedance by 10, so 3 stages is about the limit for a practical circuit.

However, this only applies if you need low attenuation and minimal interaction between stages. If you don't mind a bit of loss and roll-off in the passband then a lower ratio may be acceptable.