It grants both hexproof and indestructible, meaning you can shut down just about any form of targeted removal or combat trick
The flavor text is also stressing that the silver-bordered creature destroyed was a Donkey
And yes, seven mana is certainly a lot, even more so in Mono-White, but Elesh Norn makes sure that if you do get to resolve her, the table feels her presence immediately
There are different things which couldve made this card better it shouldve just allowed players to Special Summon Dark Magician without getting rid of itself, or the spell counters shouldve offered protection of some sort
It can also deal effect damage if Ranked-Up
Combined with the Mimi Tachikawa Tamer card, Argomon can net you loads of memory quickly