Of course, the "old, huge, tangled code base problem" is not specific to the Perl 5 internals. It is a chronic problem in the software industry

Yes, but perl 5 is an extreme case.

In any case, IIRC topaz was not a fiasco but a prove of concept that derived into the Perl 6/Parrot project. Ponie intention was to bridge XS and parrot, something dammed from the start because XS is just a way to access the perl 5 internals directly. A high level API hidden the implementation details is completely missing.

Corollary: anybody wanting to advance perl 5 seriously should forget about XS compatibility. It is a too heavy to carry stone.