Under what circumstances can impedances be simplified to simple resistances?
Unfortunately, you cannot simply replace reactances by resistances because you are then ignoring the phase shift.  In your first example, the cutoff frequency of a low-pass filter is defined as the frequency for which the output voltage is 3 dB down (0.707) from its maximum, not half.  So replacing the capacitive reactance with an equal value resistance does not yield the correct voltage output. This is because the voltage and current in the real circuit with the capacitor are not in phase, but you are forcing them to be in phase by replacing capacitive reactance with a pure resistance. I really don't think frequency dependent resistors (which are not available as passive components) are more intuitive than capacitors and inductors which are easily constructed as passive components.  In your second circuit, you completely lose the concept of resonant frequency because, unlike capacitive reactance and inductive reactance, resistances are always positive.  Thus R3 and R4 can never cancel each other out. I think you need to stick with reactances and learn to apply them to real circuits.  It will be worth the effort.