I suspect that Sony may no longer see this market segment as a worthwhile investment. It may depend on how well the D100 sells, which is very expensive given its features. There may still be enough people buying it though, such that Sony sees no reason to develop a cheaper unit in the line to replace the M10.
An "M20" that is nothing more than a refresh would be fairly boring. They would need to add real functionality, and make something competitive with recorders such as the new Tascam DR-100mkIII for me to take interest. That would cost them a huge amount in R&D and production, which is why I don't think it's ever going to happen.
The real question may be why they are discontinuing the M10, when they could just keep pumping them out with no new revision and the associated costs that would bring. Either it's not selling well anymore, or it may be still selling very well, but they want to drive customers towards the D100 which they are avoiding because of the price.