Indeed, it's challenging to simultaneously collect from multiple toy lines. It makes it harder to keep focus, not to mention the additional time and money spent acquiring items. However, many collectors are able to do it successfully, whether collecting SW figures and Lego items, SW figures and Marvel Universe figures, SW figures and Sideshow, and so on.
For me the potential redundancy of the 6" line is unappealing. I juggle a few different lines (Star Wars, GI Joe, Walking Dead, Marvel movie 6"), so while it wouldn't be a stretch to add in 6" Star Wars I would feel like I was collecting the same stuff twice. It's the same reason why I don't buy Hasbro AND Sideshow Joes, or Walking Dead figures AND Minimates. I prefer to focus on select formats/styles/sub-lines as opposed to picking and choosing from everything available.
My thinking is that if 6" figures are coming, they will be a replacement for the 3 3/4" collector-oriented line. I can't see Hasbro making both. A lot of others must be thinking the same way, hence all of the "starting over" attitudes.
Why would you assume that? Hasbro has very successful Marvel lines in both the 4" and 6" scale. DCU figures like Batman and Green Lantern have been released in both scales. Heck, even Prince of Persia and Pirates of the Carribean had 4" and 6" lines. I can't see Hasbro just dumping their existing line of 4" after 30+ years. If/when they release 6" figures it will be an add-on to the 4" platform, not in place of it.
I don't think ALL 3 3/4" figures would go away, just the figures targeted towards collectors. I won't comment on the DCU Batman and Green Lanterns since I'm not familiar with those lines, but for POTC, TDKR, Green Lantern Movie, and now Iron Man 3 the 3 3/4" lines are very kid-targeted with the 6" figures aimed at collectors. As for Marvel Universe, it appeared to have been on the back burner for most of 2012 due to Legends. I only saw MU figures stocked in October and November, compared to ML which was stocked more regularly. I wouldn't want to see Hasbro try a similar balancing act for Star Wars, and considering their apparent lack of effort with the line I don't think they would bother giving it a shot.