The issue is not with the idea itself (this is an agreed mid- to long-term goal), but with the scope of such a project vs. the available resources this year to work on it.
Refactoring a (very!) old data-block type with its (painful!) baggage of legacy code accumulated over decades is not a small task. Converting any data-block to something completely different (not an ID anymore, but in this case an attribute) is also a fairly heavy task. Combining both would be a major project, requiring months of dev-time to be completed - in best case scenario. It involves changes to the animation system, massive parts of the UI/UX codebase (editors, edit modes and operators), ID management, complex versioning… while ensuring total feature parity!
And the new attribute system has currently limitations (like lack of namespace) that would also need to be addressed to be usable for something like shapekeys (or vertex groups, which are another candidate for attributes).