From the launch of the 1200, I've consistently questioned the weight-loss figures quoted by BMW and people such as yourself who claim it is so significant.
Just how do you notice this (claimed) "lightness"? Is it easier to push around? Is it easier to pick-up when it has a lie-down? Is it easier to get onto the main-stand?
The answer to all is: No.
The "lightness" is felt in the improved power and that's the effect of an extra 15 to 25 bhp. That and the counter-balancer, which the lack of in the 1150 doesn't prevent using more of the available revs but does make the 1200 feel more 'sprightly'. The difference is only significant if you want to measure your bike against others in some sort of dick measuring competition. Used for normal real-life travelling between two points it makes no significant difference and if a 1200 owner is the sort who needs to demonstrate the extra horses, then I for one would suggest that he is not the sort of rider that I'd want to be in company with.
As for handling, steering and ride, there's little or nothing in it dynamically. The 1150 carries more weight over the front than the 1200. That difference translates as the 1150 feeling (and being) better on road, the 1200 feeling (and being) better on loose surfaces. Neither difference is significant though.
It wasn't "dumped early" at all. As mentioned by Bob (above), the 1150 was a development of the 1100 (in much the same way that the Twin-cam 1200 is to the cam-in-head version).
As to why BMW felt it necessary to replace the 1150 models with the 1200, it's simple; BMW learned from their experiences with the 2-valve boxer that if they wanted to pursue sales for higher profits they could not rely on marque-loyalty, they had to attract people who would not ordinarily consider BMW. So they compromised. They stuck with the 4-valve boxer but tried to address some of the things that put people-off, the main thing being the suspicion that BMW boxers were unexciting, plodding and only for people of a certain age (not to mention that they managed to alter the other BMW perceptions; quality and reliability

).
The answer was to give the 1200 more power than it really needs, something that most people coming from other marques were used to. The Press did the job for BMW as they too wanted more power (when do they ever not want more?) and started to openly tout BMW (and the GS in particular) as now worth considering as an alternative to the usual fare. With the Press endorsements ringing in their ears, many people now felt able to buy one and not feel too much like their dads.
That has to be one of the most ill-informed comments I've seen here.
I've never made a secret of the fact that there isn't a bike in the BMW range that I really want to own these days. So that proves your point and mine: The BMW ethos has changed, it's gone mainstream and that suits people who like the mainstream.
Personally I liked the Twin-cam 1200GSA that I last rode but it wasn't enough to make me once feel that it was significantly good enough to replace my 1150GSA. It was characterless, anodyne and too mainstream (and not without some significant faults).
There is definitely a new type of GS-owner, one that doesn't really see the bike as primarily a machine on which to travel but one to ride on fine days. This new type of GS-owner are the ones seeking more power and who will chop their bike-in for the next model or swap to any bike that might offer a performance gain. So the opinions of the here-today-gone-tomorrow owners are worth diddly-squat.
The opinions of those such as Cookie who have owned 1150's and then swapped to 1200's, I disregard because they, having forked-out so much on their 'plastic-fantastic', cannot now bring themselves to admit their error.