I don't know why people even argue about the gaining spellcasting "application" of Shapechange.
It's not going to be allowed in a game, whether it's RAW or not.
I like how my current DM replied when I said as regards Shapechange, I don't consider spellcasting to be something you gain.
DM: "Even if you did, I'd rule you'd need to rest 8 hours and prepare spells again to actually get to use them."
I guess the spell could be made to outlast that, but it does make it harder to abuse.