I've reached diamond and even champion previously. As much as I play I don't play competitive enough to stay consistent. Tonight I had more wins than losses, but tomorrow night I'm likely going to have more losses than wins.
I genuinely don't know how you could get to champ and then fall back to platinum. That is like two entirely different games. I haven't actually played the game for probably 12 months but I am certain I could jump back in and comfortably crush diamonds