The biggest problem with the open letter released by Adobe's founders is there's no there there, no substantive argument.
They warn against the web fragmenting into closed systems, but in fact the exact opposite is happening, the web is unifying into an open system with a common language; it's reaching maturity.
Instead Adobe chose to keep Flash a proprietary format. They should not be surprised that others' reactions run from lukewarm acceptance to outright rejection.
That Flash can be made to run acceptably on mobile devices, and that some web developers would prefer to keep using it rather than learn to use the open standards, doesn't even remotely constitute evidence that the future of the web is at risk unless every platform vendor were to allow it on their systems. Flash is common, yes, but its percentage of installation has already peaked and is beginning to dwindle. It is not and now will never be part of the standard, even if Adobe were to finally see the light and turn it over to the ISO or W3C; it's too late for that.