You might be right! But the way I look at it, the revenue coming in from the royalty co makes a great investment in its own right (subject to learning more).
Let's say they only manage to sell $1B per year for 7 years (which I THINK is how long Advair has been on the market), at a 5% discount rate and factoring in milestone payments and debt and some degree of overhead, I figure that's about $500M net present value. On a $3.2B company that's a 16% yield on a very conservative set of inputs. Plus you get the Theravance BioPharma for free...and they have some catalysts coming up too.
Point is, I don't think we need to worry about a GSK buyout to feel pretty good about the situation as it is.