I also have had the Space 1999 series experience. Unfortunately this sort of thing happens all to often from, it seems, all the companies.
Not only have Space 1999 gone slim, but in another package there is a remastered version, region two last I checked, and it sure looks so much better than the package I, and it sounds like you have.
For the most part I have removed myself from the mind set of replacing for the latest incarnation if a package is re-released. Just tired of the treadmill.
There is of course that extremely rare occurrence , but it does stay rare. A greatly improved Space 1999 visual could be that rare occasion, but not for the slim.
As to Star Trek, once I purchase a DVD package, that will be it for me. That is one of the reasons why I have not bought them yet. Plus they have been much over priced.
Star Wars is a great example to wait. I am interested in the original three, not all the recent stuff. Anyway I figure George Lucas has to be dead about ten years before the final version has any chance of seeing the light of day, and the argument of who shot first will be settled!
But for me it was Han!