I feel that is a very conservative estimate. The demand for transit in the deep southwest is there but due to North American attitudes towards buses it has yet to be unlocked in any meaningful way.
With Heritage Valley having a large, free and dedicated park n' ride (leaving aside opinions on the highest and best land use yada yada) and ETS reallocating buses freed up from not having to essentially deadhead 10 different routes to Century Park all the time, there should be substantial opportunity for growing the network in the southwest.
If the current Capital Line is able to produce about 100,000 raw riders per average weekday I would not at all be surprised to see this extention add another 25,000. Especially once the province gets their head out of their ass and builds the new southwest hospital.