Can there be legitmate use of purchasing post promotion?

Probably not at this point to be honest. The audience is just too small. Who are you promoting to?

If the issue is just getting some early votes to lift the post out of the noise floor of unrewarded (or minimally) content, we have curation rewards for that, and I fully expect that with curation rewards being doubled there will be a surge in both active curation and delegation to curation initiatives (absorbing some of that delegation you were concerned may no longer want to support unused bots).

Overall the ecosystem is still evolving after these changes and we shouldn't expect everything to happen overnight. In time, yes, I do think there is a role for "promotion" broadly defined, although I'm still not really sure what is the ideal structure for that will turn out to be. Assuming it looks just like it did in pre-HF21 is probably a mistake.