"then we will see another coin go up, and the difficulty of ethereum would decrease aswell"

That's actually not correct - Ethereum difficulty is not related to the amount of hashing going on - it's completely disconnected and growing fast now as they had intend to push miners off the currency so that they can move it from proof-of-work to proof-of-stake, so the difficulty is now rising exponentially.

However, they did decide to delay POS until next year, so hopefully they'll crank down the difficulty before the miners all leave before they're ready.