i agree with above about the Sigma 70-200 OS, it's fantastic, and I feel that the bokeh is a lot smoother and "dreamier" then the Canon version. Though, not as sharp at 200mm, but considering I got the lens under 1K I'm very happy.
But, also with the author here, I'm still on a T1i too, and I might just get a 5Dmk2 for cheap as the 6D, though not THAT bad, doesn't look like a camera I'd shell out 2K+for and get effective use for the next 4 years or so. It has more limitations than advantages, with the only real advantage being price.... which isn't even great for what it is so let's move on.
Switching systems no matter how much equipment or experience is just unwise, not just financially, but when you jump once, then if ever the previous system provided something you'd like then you're blaming yourself for not sticking with them, and might heavily consider switching back. The grass is always greener... even though right now it may really be green, a drought could harm Nokin land soon and the game could change all over.
Though, even if great things come to those that wait, if there's a feature/piece of equipment you need that just isn't available then of course, those events/sports/weddings/stories aren't going to wait, so switch over like all those guys did with the D3. But switching systems over an "entry level"offering doesn't seem like a good decision at all,
unless the goal later is to move up and collect lenses in the meanwhile with an affordable body...
I agree with you in general terms, but there are some peculiar things to consider in this situation:
1) D600 and D800 are so well specced to be quite a good investment for the future too, especially if you're a low-iso shooter.
2) They are cheaper, which makes up for the losses of switching system. And if you upgrade from a crop, you lose your crop lenses all the same, which makes it hardly different.
3) Sigma and Tamron provide very good glass, which makes users less dependent on Canon and Nikon lenses.
4) The D600, though being entry level, is very well specced. The typical user gets 90% of a 5D3 for 60% of the price.
In this peculiar situation switching systems is particularly soft.