There are several factors that inform the choice for a 50W rating instead of 20W.
People who install aftermarket speakers may be likely to also install an amplifier. Those amplifiers can generally deliver much more power than the 18 W a headunit is limited to. If you were to design your aftermarket speaker to have an 18 W RMS limit, you'd miss out on that section of the market.
Another factor is distortion (clipping). 18 W is so little that people quite often drive the headunit at its maximum power, so the amplifier will start to distort. Distortion means the sound waves contain more energy: the sound wave looks like a square rather than a sine, so RMS is more than 70% of peak power.
The distorted sound wave also contains extra energy at high frequencies. This energy is routed to the tweeter, which isn't designed to handle it.
That extra power has to be dissipated by the speaker. Some of it gets turned to sound, but most of it turns to heat.
This heat will eventually melt the insulation on the moving coil that drives the speaker membrane. That will cause a short and/or jam the coil in its gap, making the coil and membrane unable to move.
A 50 W speaker has thicker wiring in its coils than an 18 W speaker, so they'll take longer to overheat.
It is not necessary to match power ratings between the amplifier and speakers. In general, you'll want an amplifier with a higher rated power than the speakers, because then you can be sure the signal remains undistorted and clean until you get to the speaker's limit (instead of the amplifier's limit). At that point, the distortion becomes clearly audible and you can reduce the volume before damage is done.
You can ignore the peak and PMPO ratings, those are useless. The only rating worth comparing is the RMS rating.