You are projecting Microsoft's behaviour over a decade forward based on the current personality of the company today.
However the company's personality could easily change with different circumstances. Compare, for instance, the reputation of IBM today and 10 years ago. Therefore you should assume that Microsoft won't necessarily act according to any current strategic vision, but instead will act as normal economic principles dictate.
When it comes to profit maximization from a locked in and not growing market, standard advice for vendors is to raise switching costs as high as possible, and then slowly increase prices until people scream and switch. (Suggested reference on usual lock-in strategies, read Information Rules.) In that scenario, Microsoft has every reason to keep Mono from being a viable option.
They are likely to switch into this mode fairly shortly after they accept that they aren't going to grow the .NET market much more and .NET doesn't fit into any particular larger strategy. I would not be surprised to see them make this transition within a 5 year window.
Cheers,
Ben