I think that sometimes we think just in terms of the raw price of the incidentals rather than the effect of reducing demand for incidentals.
The main reason Ryanair is so cheap is that they have the fastest turn around time in the industry. This means the utilisation rate is far higher. Part of the reason they're able to turn around so quickly is that they take less hold luggage (so unloading/loading is not holding them up), un-allocated seating means they get to overbook/bin pack better, worth thinking through the second order effects.
Ryanair haven't had unallocated seating for years (I don't think it's generally allowed anymore); if you don't choose a seat you just get one allocated for you.
Or to look at it another way, they're perfectly happy to let you stand in a queue, outside, on the runway, in the cold/heat, waiting for them to unload/load the plane that has just landed and is often already late.
After reading their SEC filing I would push back on that slightly.
I think the main reason Ryanair is so cheap is that they get insanely good deals with the airports because they bring so much business to the region. Ie Alicante, a secondary airport, is somewhat tourist-dependent.
Ryanair makes a deal with the municipally-owned airport and says hey, we're going to bring thousands of Brits and Germans to you (with full wallets). The municipality gives a good deal in return.
There's also the 1.5x revenue multiplier from ancillary revenue.
Finally, they only own one type of airplane. Makes the maintenance etc easier.