To answer your last statement, no, that isn’t always true and also a dangerous propostion. And, in fact, sometimes it is a good thing to have the prior version sticking around in case there are issues. In the case of RW, I have some sites in RW5 that can’t be brought forward to RW6 just yet, so, I still need RW5 around. Yes, lots of software will replace previous versions, but not in this case, and that is a good thing.
Your second question: it isn’t the “same” program. We “version” applications to keep track of which is the latest. The computer doesn’t care what the version is, all it cares about is whether the instructions provided by the program are valid. Imagine a novel, of which there are two editions. The first edition reads one way, but, the author had a change of mind about some sections, and re-wrote portions, producing a second edition. Both books are still readable, just one is more “current.” Thus, they are not the “same” book.
“All other versions have done this seamlessly.” I’m not so sure about “seamless,” however, as the application versions were meant to actually replace the previous ones, it is possible to see it that way. RW6 represents a different paradigm (though, to “you” it might not be quite so much), so, things work differently and the projects have to be converted to the newer sandwich format.
In RW6 the method to bring applications forward is as per my previous reply - make sure you have all the current/latest themes/widgets/stacks (RW6 is heavily stack-based, NOTE: those items must be the RW6 versions) that are in your previous projects, then open your previous project (the RW5 file) in RW6. RW6 will attempt to convert it, and warn you if anything you have isn’t compatible.
Now, you save the resulting file. It will be in RW6 format. Your old RW5 file will still exist, and you’ll also have the RW6 file too.
So, now, you try that. If you get any errors upon conversion that you don’t understand, post the screenie here and the group will help out. Otherwise, if everything is up-to-date, you should have no errors and be able to work on your project as an RW6 project from that point forward.
This has to be repeated for each site you created prior to RW6.