Reserved seating works for a wide variety of circumstances - Broadway theaters, restaurants, etc.
It is just a matter of keeping track of who is where. As long as you are in your seat all is good - but if you want to leave to you have momentarily "check out"
Is it not the same with a restaurant? While your friends are there you are checked in even if you leave, but if no one is there and they don't know what your plans are for coming back - you are checked out, whether you leave a book behind or not.
It's just a matter of them caring enough to police it.
The fact is that there are a certain number of pool "bullies" (if you will) who know how to stake their territory and want to keep the system as it was. They are the ones protesting the loudest. I would be willing to bet the vast majority of currently unasked people would fully understand the need and the procedure for a system of tracking deck chairs with no problem at all.
To me this is one of those cruise paradoxes. If certain deck chairs came with certain suites people would never stand for it. After all, everyone has equal access, etc...
But currently deck chairs are going to the early risers who know the trick. And as this gets more publicity even more people are going to start reserving chairs from the first thing in the morning on all ships without a system - just because they can.
The systems are coming to all ships eventually because once something gets started there is no stopping it.