Transient analysis of a step down converter with control loop
If I use an ideal opamp, it would saturate even though there is a feedback loop. Why?

Isn't this just a result of the fact that the output voltage = \$V(in)*Gain\$

You can check this by increasing the gain by a factor 2 and checking if the (what you think is the) saturation voltage also increases by a factor 2.

I would get a PWM output whose frequency will not be fs.

I think you should get a PWM signal with frequency Fs. Have a look at a datasheet of any switched converter. There is a sawtooth or triangle generator which sets the switching frequency Fs.

I think you also have to pay a bit more attention to the expected default values of the voltages in your loop. Like: what DutyCycle do you expect when the converter is working and what will then be the PWM control voltage ?