flatlander

If we assume that 90% of seats have an overhead bin above them they can use and the seat pitch is 30" (76cm), then that's about 23cm x bin height x bin depth per customer. Meanwhile a maximum size carryon bag is 35cm wide, so in the best case about 2/3 of passengers can bring a maximum size bag on board. In practice there is unusable space, the bin length is not the same as the bag width and so on, so about half the passengers are likely to be able to bring on a maximum size bag.

Clearly if most passengers are not going to pay or feel they do not want to check in a bag yet still want to bring a maximum-size carryon for their trip, this is not going to work out how all of them would like it to work out. Look at any of the leisure routes where people typically take a longer trip.

Meanwhile, in Club Europe there is almost enough space for everyone bring on board a maximum size item, so as long as the bins over Club seats are kept for Club passenger baggage, they won't have a problem and boarding last will be fine for them. I'm afraid I do note that quite a few of the board-last-no-problems advocates either always travel in Club or travel with very little in a bag or both. For those where neither applies, boarding last means being separated from your bag that you aren't entirely intending on being separated from.

(Personally I check in a bag and bring a half-maximum-size backpack on board with all the things I need to stay alive, to work at my destination, that aren't insured in the hold, or that I want en-route and check the things that won't pass security screening and the rest into a bag that goes into the hold. Even then, I try not to board among the very last, or there may well be little room for my 20-cm-wide bag).