If X360 successor came out in 2010, that would indicate that the X360 lost out in the market share battle and that they were still losing money.
Because if the X360 was still selling well in 2010, MS should be able to rake in the bucks with a cost-reduced design.
Not necessarily. Sales can begin to significantly slide after year 5, and there is no guarantee that
a.) N6 would not release early due to Wii being primarily a GCN upgrade
b.) Sony moving to a 4 year cycle if MS eats into their market share
By going 6 years they also run the risk of co-launching the same year as Sony if Sony runs a 5 year course before PS4.
On the other hand if MS released in 2010 with a solid piece of hardware they get yet another year (or more) lead on Sony if Sony goes 5-6 years. Sony could put Sony in a difficult position where they have to decide how long of a lead they give MS. 18+ months could be detrimental.
Of course they could go the milking route and assume Sony will go 5-6 years and still be able to come out a year early in 2011 and get those nice cheap sales. A lot may depend on
a. How long it takes developers to begin really hitting the wall on design with the consoles
b. How new technologies and process node development are going and are projecting over the lifespan of the device
c. Sales of current consoles and games
Anyhow, too many assumptions to arrive at the conclusion that a 2010 launch means MS has "lost" and is losing money. I don't think they are going to win market share (they don't WW it seems either) but their goal is to make money. If money is the issue I would actually expect a 2011 launch for the reason you note: the 360 will be very cheap to make in 2010/11 and will have a lot of software. A new console launch is very expensive and guaranteed lost.
But we will see. All conjecture at this point. I have to believe that 27M in the US alone for MS would be considered "good". That is 11M more in the US than last time around (plus additional sales after 2010 unlike the Xbox that has pretty much ceased production). With those sort of numbers MS could get close to doubling their install base this generation which would have to be seen as a win for MS in most perspectives.