Unfortunately, that's just a sliver of spectrum; the entire FM broadcast band is about 20 MHz wide, equivalent to a single 802.11b/g channel.

The entire AM (MW) band is little over 1.1 Mhz wide.

Keep in mind that comparing bandwidth between bands that are very far apart isn't super useful. You can easily select down to fractions of 1khz at 1mhz but not at 2.4ghz so 1.1mhz of bandwidth means something different at UHF vs HF/MF.

My memories about radio are a bit faint, so forgive me if I am wrong, but the fact that you ate able to discern narrower channels does not mean that you can transmit more information, doesn't it?

As far as I remember, the upper limit to the quantity of information you can send is solely set by the bandwidth, and is not dependent on the carrying frequency.

That's mostly right, the theoretical limit to the amount of information you can carry over a channel is set by the bandwidth and the channel signal to noise ratio. See the Shannon Limit [1]. I should add, however, the lower frequency you build your radio to operate on, the narrower your channel usually is. Antennas and filters tend to only be well matched over one or a few narrow frequency bands. These bands of good impedance get smaller as you go down in frequency, logarithmically.

[1] https://en.wikipedia.org/wiki/Noisy-channel_coding_theorem

There's also the limitation of oscillator precision IIRC. 1ppm deviation is 1hz at 1mhz and 1khz at 1ghz

Modern frequency synthesizers can easily do a few Hz step size up in the gigahertz range and be within a few ppm. That doesn't really matter that much though, since modern radios (at least these digital ones) are built as SDRs. They can shift the signal around and select whatever band they feel like.

Sure, but that's on the receiver side for carrier recovery. Wouldn't it be problematic if you mashed a bunch of channels/allocations sufficiently close together that they started wandering on top of each other?

Modern DAB/DVB receivers are usually zero-IF, the wanted band of frequencies is mixed down to baseband and filtered with a sharp cutoff Surface Acoustic Wave filter then sampled with a fast ADC. Selectivity is then dependent on DSP.

On the transmit side, you have to carefully combine the outputs of your various transmitters with a complex filter network, which at high powers looks something like:

http://admin.mb21.co.uk/tx/userimages/8448proc20120403120238...

The spectrum used by FM/UKW radio really doesn't matter. It's not particularly wide, neither supplies much bandwidth.

I frequently listen to a classical music station using a radio from the 80s and the audio quality is very good, apart from the noise floor I'd call it almost close to a 128 kbit/s MP3. I know because I own (CD) copies of a whole bunch of recordings the station has in their inventory as well.