grassycow i agree with your other points but you're overstating Sableye as a whole. I don't really think that Sableye outclasses Mew tbh. Sure, it has Prankster Taunt, but that's only really more effective than Mew in the case of leads faster than it, such as lass and Azelf, as Mew outspeeds a large portion of the common hazard users in the tier anyway and handles them much better. Sableye's also not as good as Mew because it just flat out loses to two of the most common walls in the tier, Florges and Aromatisse, who Mew beats 1 on 1 unless they're using Toxic (granted it takes like 3 years for Mew to beat Aromatisse but still). While Sableye does have an arguably better defensive typing, its bulk is just piss poor compared to Mew, as Sableye generally has a very hard time with special attackersr such as Infernape, Shaymin, and NP Lucario. Sure, Sableye can handle physical attackers /somewhat/ better due to Prankster, but the fact that Mew can handle physical attackers only a little bit worse, and doesn't have to worry about being taken advantage of from both attacking sides at the cost of a little bit less speed makes it better than Sableye. Really, the only advantages of using Sableye over Mew is winning against common leads and fast stallbreakers, namely Mega Aerodactyl.
I'm not opposed to Sableye moving up; in fact, I think A- or maybe even A is good for it, but A= is just too much for a Pokemon that has much more noticeable flaws than Mew.