Flex Queue Restrictions in higher tiers

Hi,

Could anyone clarify the queue limitations for Flex Queue for Diamond I, Master, and Challenger players?

This article on Riot's page mentions that Diamond I and Master players can only queue up with 1 more friend while Challengers can only solo queue. Is this for both Solo Queue AND Flex Queue?

I know it seems obvious but if Flex queue was made for groups of 3 or 5 players that want to play competitively, why would there be such restrictions that far in the game?

If I have a group of friends I'd like to play competitively with and we made reaching the highest rank in the game our goal, why would I play League of Legends if once we reach Diamond I, we won't be allowed to play together anymore? Challenger is that highest rank but we're not allowed to reach it as a team?

Which brings me to my next point. I don't know if these restrictions are made to balance the "end game," or to prove individual skills when in Challenger tier, but if we have 2 separate ranked queues with separate ratings, one of them being for solo players and another one for groups, why would we switch the purpose(?) of Flex Queue at the last tiers to resemble Solo/Duo Queue?

I'm pretty sure that solo players wishing to prove their own skills would stick to Solo/Duo Queue.

P.S.- As a biased comment, I liked having one rating everyone focused on, whether in a group or solo. Probably because I usually played with a group of friends.