Just read an article phys.org/news/2017-06-ryanair-random-seat-allocation-oxford.html
Tests were carried out where groups of 4 checked in without having pre-booked seats. All were allocated middle seats. Apparently the odds of doing this are 1:540,000,000 vs 1:45,000,000 for winning the UK Lottery.
According to a statement (www.independent.ie/irish-news/ryanairs-random-seat-allocation-not-random-scientists-35880337.html) they hold on to aisle and window seats until the last minute as people are more willing to pay for those. I guess checking in at the last minute is the sensible way to go if you don't want to pay. At least then you've a chance of getting aisle or window.