I agree the performance increase would be great, but I think it needs to come gradually to MRI. E.g. trying to do anything fancy with the old AST-based interpreter would've been pretty pointless. After YARV, it is probably starting to get more attractive, but at the same time they've added method caching which gives a decent amount of the benefits. A vtable will still get faster, but it might not be the most immediately expedient way of speeding things up vs. e.g. Sasadas latest project of adding a generational gc.

Regarding Rubinius, writing compilers for dynamic languages is hard. Most textbooks you'll find cover techniques most suitable for statically typed languages (the best resource I know for starting to catch up on compiling dynamic languages is actually the Self papers). So you need more than an unusual level of interest in writing compilers to be likely to try to tackle a language like Ruby which is tricky even for dynamic languages (e.g. my favorite pet problem to meditate on: What constitutes 'compile time' vs. 'runtime' for ahead of time compiled Ruby?), and even more to actually persevere until you start getting proper results where you can get decent results in days with a simpler language.

It's made worse because of Ruby's horrendous grammar. And I mean that from a compiler writers perspective - as a developer I love to use Ruby to a large extent because the complexities of the grammar means it reads and writes better 95% of the time. But MRI's bison based parser was 6k-7k lines with ugly parser/lexer interplay last time I checked... There are full compilers substantially smaller than that for other languages...

To me, these complexities are part of what makes it fascinating. I firmly believe you can parse Ruby fully with a much, much simpler parser for example. A lot of the ugliness can be abstracted away, and C parser code is rarely good examples of succint code.

I did start playing with MRI years ago, specifically the parser, actually, and started chopping out redundant pieces, but got frustrated and bored with it. That's part of the problem - it's one thing to play around with a toy compiler like I've done, and another entirely to put in the effort to push a major change to MRI through to production quality given the number of years of accumulated history encapsulated in it. Doing the latter as a hobby is a daunting task.