Safety - imagine trying to evacuate an airliner when there are bags on the floor in virtually every row?
Either the bags are dangerous or they aren't, doesn't matter how many there are. In my experience, Ryanair staff are amongst the most efficient at making sure they are stowed properly.
and airliners are palpably not designed to manage this much luggage within the cabin?
Oh please! Airliner design has evolved to take this into account. Remember when there was just a shelf, no bins?
If Ryanair's practice was not safe, it would not be happening, period.
If there are modes of public transport where bags are a problem, try evacuating a train or an urban bus in the run up to Christmas. Another one for PTDRuNe.