The effects of DTrace's problems
I was a little confused at first about the language and documentation issues [with DTrace]. Usually language discussions [...] are intended to pick one over another, but in this case there is no other option to choose from. So if these problems are raising the barrier to entry for some people, and they aren't entering, then what are they doing? Leaving system problems unsolved?
There are two answers to this.
The first answer is that DTrace is two things at once; it is both a way of diagnosing problems on Solaris and a potential way of attracting people to Solaris (and all its variants), to continue to use it, and to use it for more things. Let us focus on the latter thing for the moment. When and where DTrace is hard to use it becomes less attractive; at the limit, if you feel that you can't really use DTrace for anything it ceases to become an advantage for Solaris at all. If you want Solaris to succeed as an OS, this should matter.
(I think that the theoretical advantages of having DTrace may have been oversold in general. As a pragmatic matter I think that most people don't expect to have system problems (they expect the system to just work), so I suspect that they drastically discount the availability of good diagnosis tools because they expect to not need any. People who know that they are running at the ragged edge of performance will have a different opinion, but many people are not in this situation.)
The second answer, put simply, is yes; sysadmins are leaving system problems unsolved because DTrace is too hard to use. Not the big crippling problems, of course, because those are the problems you have no choice about solving. But smaller problems, the little glitches that happen sometimes or the relatively low impact performance degradations? Yes, some of them are going unsolved. Also going unsolved are the problems that people don't even know they have because they've never looked, the ones where people have no idea that something is actually wrong and their system could work better with some changes. DTrace being hard(er) to use is especially damaging to the latter because of course if you don't think you have a problem, the cost to benefit ratio of looking into your system appears infinite.
(I've argued that this is not actually the case, but I think it's at least a very hard thing to sell. Especially to overworked sysadmins with other issues to tackle when you are asking them to invest a significant chunk of time.)