[Radio Wrangler]

Even if images, intermods and other ghosts have no FM on them, they tend to cause swirling noises, pumped-up noise floors and other things which spoil the signal to noise ratio of your wanted signal.

If you try to make a narrow tuned RF circuit, its loss increases the more narrow you try to make it. So you soon need extra RF stages to make up for the loss. Analye this carefully and you'll begin to see that because the amplifiers also need selectivity to protect their inputs form out of band signals, the optimum structure becomes a series of alternating selectivity and gain stages to give the best compromise between noise figure and non-linearity effects.

So the old fashioned IF with multiple gain stages with selectivity between them was actually quite good if done properly. What supplanted it, having all the filtering in one block right at the beginning of the IF works, but has disadvantages in broadband noise, so filters got added in later in the IF amplifier chain to stop the detector convolving-in noise over the wider bandwidth of the IF amplifier stages.

Limiter amps are not perfect and don't strip away all AM problems. AM to PM conversion is a vice of theirs and converts any AM to a little phase (frequency) modulation, which gets demodulated and adds into the audio.

David