I don't believe it's impossible, although it's certainly a large undertaking. TruffleRuby already optimizes some "slow" features of Ruby quite well. E.g., it's able to inline blocks and JIT compile metaprogramming features. I haven't really kept up with all that Crystal is doing these days, but if you can optimize the hard parts of Ruby, you eventually just get into the traditional trade-offs between AOT and JIT compilation.

(Full disclosure: I work on TruffleRuby, in case that matters.)