The kernel by itself is pretty much useless. Whether or not something is good for the kernel (IMHO) quite obviously needs to take a back seat to whether or not it is good for the users. The kernel is also "weighed down" by the more-or-less strict adherance to API compatibility, yet I don't think anyone would argue that this policy should be dropped simply because the kernel would develop faster.
Or how would you like to run an OS that has just the linux kernel? Yes, this is a stupid strawmen-argument but it's the logical conclusion to your argument. Obviously a line has to be drawn somewhere between kernel benefits vs. wider benefits, but simply ignoring the latter is a rather odd suggestion.