This comes up in military hobbies... 1:32, 1:6, 1:12, 1:18... There's been a lot of scale choices, and that's only 4 of many more (1:35, 1:16, etc.).
I think for many people, changing scales is ultimately collecting something wholely separate and new. I know I view that with 1:6 and 1:16-1:20-ish (Star Wars is technically 1:20 for the most part). For me, that's a whole new toy line... It would be tantamount to me going and saying I'm buying Hobbit/LOTR figures now.
Like Jeff said, I'm invested, heavily, in the 3.75" line and it's many hundreds and hundreds of figures. Army building alone, I've got a ton tied up in this. It's become the scale I like. I WILL pick up a 1:6 figure here and there. I DO collect other lines (Lego LOTR/Hobbit), but for Star Wars I'm a 3.75" scale guy or nothing for the most part. I am dedicated to this.
I view this as nothing different than the Unleashed line... neat looking at the time, but it was something else to take up room that is tough to justify.
And I do see it, if they are somewhat abandoning 3.75", as basically your choice is to start over or not. Anyone can pick up what they want, but that usually leads to more and more, and before you know it, now you have a 3.75", 6", 12", and beyond collection of crap.
My crap collecting needed a cut off point, and my point was when they decide to change scales. At that stage, I'm out. I didn't buy Mighty Mugs, and all kinds of stuff like that, and this is just another thing I think I'm very much not into not because it would be bad... I think it might be very nicely done. I just know it's time to turn the faucet off with things like that. I did the same thing with Clone Wars actually. Hitting much closer to home, I bought only what would look ok on a realistic shelf and not too toony, and that was fine by me.
My only anti-6" opinion is that, as with anything, it takes away from what I actually DO like. So has everything else they've ever tried though, when you think about it. From 12" figures to Mighty Muggs and those beans, it could all be bs'd that it took $ from the main 3.75" line at the time. If 6" kills 3.75" then so be it.