What exactly is the benefit of having multiple compilers? I'm not well versed in the area, but it seems like having one compiler is better from the perspectives of standardization, having a single way of handling undefined behaviors, etc?
Actual standardization (as in, there is a document describing the what's supposed to happen) is helped by multiple implementations because their conflicts will help discover unclear parts. This in return helps new implementations get of the ground if there's a need to produce them. Some standards groups require multiple independent implementation of something to exist before it is allowed to be released as a standard.
Different implementations might be different enough that some changes are easier to make in one than the other. This makes it easier to test these changes, the other implementation(s) can then decide if the gains are worth their effort.
It provides an out if one project resists changes for human/political reasons.
I wouldn't be surprised if there's a few unusual architectures around that use Fortran but aren't handled by LLVM.
That's workable, eg for Python (and in practice, Haskell). But is seen as less than ideal.
The rest of your comment is... way too political towards gpl.
Apple pushed clang as a competitor to gcc much later, closer to 2010-ish.
https://gcc.gnu.org/ml/gcc/2005-11/msg00888.html
Doesn't seem to add up with your proposed timeline. Care to explain?