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.

LDC and GDC are both very good compilers.

Technologically D suffers more from having a single frontend implementation than having multiple backends.

Just wanted to point out that this is just a proposal, so if enough people voice their opinion, which many have already done, I am sure the core team will adjust their approach.