PPAs aren't a replacement for the package management system. They're just another repo. A transactional packaging system would be cool, but it's a much bigger change than implementing PPAs.
They fulfill different goals. I've reached the stage where I don't always need the latest software—I'm more concerned with something stable that receives prompt security patches. For this reason, I've been sticking with the LTS Ubuntu releases rather than upgrading twice a year (though Kubuntu 15.04 has been awfully tempting for me). I guess I'm getting boring with "old" age—I built my systems following Linux From Scratch back in the day; now, I just want a system that works.
A transactional packaging system would definitely ease the pain of running newer software, but I don't want to take the time to try it—I'd rather stick with the release cycles provided by the distro as a whole. In this situation, PPAs are great for when I want to install software that isn't in the official repos (or is at an older version).
Both have the goal of to make it easy to get newer software, or software from other sources, within the same management interface, and without breaking all the other packages of the distro. But transactional packaging systems does get to those goals, while PPAs just maybe get there, for a time, but always turn-out breaking something.
But if you are using PPAs to create different lists of released software, so you can keep you stable software while everybody else wants to move to something newer, transactional packaging does not have that goal. Although it makes the job of maintaining such PPA much easier.
