Staravia—>Staraptor

who would genuinely get excited for a starly CD?

Maybe not a CD, but potentially a Breloom situation where they have a Sinnoh event and any Staraptor you evolve/encounter during the event will know Sky Attack. Having said that, they already gave Honchkrow Sky Attack without locking it to an event and it has higher DPS than SA Staraptor would.

Or another way to look at it is if you really *need* to evolve your Staravia. You already have the dex entry and it's not like Staraptor is some meta-defining menace.

To tell you the truth it's extremely unlikely such an event would take place. Feel free to do as you like but i dont think holding on to it is ideal (if you are pressed for space)