Great write up and it's nice to see what we're getting in context with what there is available.
To one point:
I'm certain the answer comes down to the budget for the project and the desired price point. Even though it's far from complete, five discs is still a lot of content. I'm sure the record company also sees $60 as a tipping point. That price and volume of content will appeal to a wider array of buyers. I'm sure they're correct to believe that if they had pushed the price up to $100 and included 8 or 10 discs the sales would have seen a sharp drop.
I don't think it would have made a big difference. If anything, more of the bigger fans would be willing to buy a complete set. What they've come up with is neither here nor there, half way between a one or two disk collection of highlights and the full set. Yes, it's 100% consistent with their past approach, and it's not surprising in the least, but it is a bit of a shame. Christian's nice post showing what available and what's included put it in nice context for me.