Between the unproven nature of the patents in the case of third party implementations, the uncertainty of how enforceable software patents are at all given recent court precedence, the RAND terms, and the Community Promise, one can hardly say that Microsoft can "pull the plug" on an implementation of ECMA CLI. If anything, it would seem more protected from patent litigation than the average bit of software.
The uncertainty is enough for them to "pull the plug" for all practical purposes simply by dragging people into court. If SCO were able to keep IBM and Novell fighting them in court for 7 years on the basis of ridiculous unfounded claims, then how long do you think a well funded company like Microsoft can keep third parties spending money defending themselves for?
MSFT could drag any software company with a piece of software of significant complexity into court over patent infringement however spurious those claims may be. So nobody should ever write software, right?
The parts of the ecosystem that are in danger would be things specifically _not_ included in ECMA CLI. For example, Mono implements API's that Microsoft includes in .NET but not in any of the ECMA specs and not covered by the Community Promise. This is because software written to run in the .NET Framework generally uses these unprotected API's and one of the goals of the Mono project is to run .NET apps with as little hassle as possible. So, they implemented these API's for better or worse.
Using the ECMA CLI does not include implementation of unprotected (by RAND terms and Community Promise) Microsoft API's. This constant threat doesn't really apply. No, Microsoft could not pull the plug on this.
