Drawbacks of RF downconverting?
If downconverting is not that cost-expensive (Uhm, presumably. I have little knowledge in RF signal processing) and preserves the information, then why doesn't every RF systems in this world use one?

There's many other things that can influence that choice. Sometimes your signal of interest is too low in frequency to be transmitted easily or filtered and you want to convert up... anyways:

There are tons of transceivers tuned to a specific range of frequencies like L/C/S/X/Ku/..., but wouldn't it be much simpler if we, say, have only one kind of transceiver and use an LNB to tune the incoming signal into the transceiver's frequency?

Well, you'll find a lot of these systems are indeed using intermediate frequencies, so, basically, what you propose already happens (in the shape of superheterodyne architectures, or by zero-IF; some satellite transceivers even have IQ demodulators, only that their I and Q branches are themselves not actually low-pass signals, but IF signals, which get further filtered and then finally downconverted to baseband II, IQ, QI and QQ and reassembled into single signal digitally... don't think too much about that, it's an extreme example, but I added it to show that specific requirements can lead to very particular designs).

So I guess there are some kind of drawbacks, whether it's about cost or decoding problems, seeing people still make things like expensive SDRs. What drawbacks does downconverting have, if any?

Don't know what you mean with &quot;expensive SDRs&quot;: If you buy a 2€ integrated sub-GHz RF-transceiver-microcontroller, that's a superhet SDR. If you buy a &lt; 1€ FM receiver IC that delivers digital audio: that's a superhet that does passband subsampling, with a high probability. You buy a Sigfox chip: ha! Mixture of direct synthesis and superhet, often with peculiar non-uniform ADCs.
If you buy a 3€ wifi chip, that's a quadrature downconversion SDR architecture with application-specific math accelerators. Same goes for every LTE phone, and twice for every cellular basestation of the last 20 years.
So, I think you're on the right track: for a lot of low-to-medium bandwidth, things are SDRs with down/upconverters. Only when the signal is already, for all practical means, already in baseband relative to what an affordable ADC can do will you do no further downconversion. (that's why there's a run for direct sampling in HF amateur radio: it's really no big deal to directly sample the 30 MHz bandwidth from 0 Hz to 30 MHz)