The Royal Scam
It doesn't matter who's "fit". Princess Celestia is the absolute monarch of Equestria and rules with an iron hoof.
Any attempt to replace her will be dealt with in the most ruthless manner possible, potentially including banishment to the moon for a thousand years. Don't think she wouldn't do it again, because she totally would. Just try her. And don't even think about making her call on her alicorn posse to put the smack down, because they be gettin' all up in your business yo.
Meanwhile, as long as Princess Luna doesn't rock the boat, she gets to rake in all that sweet princess pay in exchange for sleeping all day and spying on ponies' dreams. Nice work if you can get it, and good luck trying to talk her out of a cushy gig like that.
So the question is moot, although I did take the liberty of informing the Royal Guard about this seditious little discussion as a precaution. Let's just say when "Sunbutt" finds out you you've been talking trash about her, I suggest dressing for cold temperatures and low gravity.