Earlier quoted context omitted.
Having the choice for advanced users is a plus but I think having to choose is a big negative.
Most importantly it results in a significantly increased workload for the maintainers, and spreads out effort over a bigger area. So the most likely outcome is a language with multiple mediocre compilers, instead of one really good one that everyone works on. Which is the exact opposite of what the intention behind this proposal is, which is to make the language more maintainable.
Technologically D suffers more from having a single frontend implementation than having multiple backends.