Disney are not going to spend phenomenal sums of money expanding a transportation system when there is going to be no benefits in doing so - except for delight among monorail fans - and long suffering members on these and other boards when the expansion rumours would finally stop (for a while at least).
The monorail is fine for transporting a relatively small number of guests over relatively short distances as it does just now. It would not be practical to expand it further really. If you think about it as well you would have to invest in bigger trains potentially to carry more people over a route - such as TTC to MGM via Epcot - if the buses for such a route were removed. And it's not just the trains - it would be the additional depot for these, ground work to secure the new pillars...the list goes on. The costs would just be huge.
The bus, for the moment, is the most practical way of moving people around - especially at peak times. The buses disney now order are configured with less seats to allow more standing room and a slightly higher capacity. It is a flexible system and doesnt need to be stopped depending on the weather, or firework displays as the monorail is. Disney can also charter in buses as and when required to cope at times such as - easter, july 4th, thanksgiving, christmas / new year. I think you'll find the investment in the transportation system over the coming years will be in electric or more eco-friendly buses.
There really just is no point in expanding the monorail. The money would be far better spend in terms of new attractions and upgrades to existing attractions and hotels. Monorail expansion is not going to bring additional guests to the parks - however investment in new and existing attractions will - and that is where Disney will spend the money - on something where they will see a return over the many years to come. With the monorail expansion this will just not happen.