This problem seems to indicate a tremendous failure on the W3C's part to anticipate the actual consequences of vendor prefixes, or if they actually anticipated them, a failure to act. Despite the fact that Quirks Mode is not identical, it is a similar enough case (And one that should be familiar to every browser vendor) that ignoring its example when considering vendor prefixes suggests either willful ignorance or an…
Here, the problem isn't one of capability. The other browsers can (and have indicated that they will) implement the webkit syntax and feature set.
The W3C is simply clinging to a failed spec "because its the spec". As was made clear in the OP's plea, the W3C needs browsers to observe their specs to remain relevant and avoid circumvention.
If the webkit extensions aren't technically or legally barred from implementation, there's no reason that other browsers shouldn't simply adapt. Aside from the W3C's own self-interest, the only "problem" here is that the market is dictating the solution, instead of a panel of self-professed governors.