I think the ability to de-rank etc is vital if we're to avoid the higher leagues becoming watered down with players who basically end up being fodder and quit playing ranked anyway. Someone who belongs in silver or gold would have a MISERABLE time in plat or above.
When you get to the top of the scale though : it's common sense that MOST people in every game, are going to lose. Within each league there are still going to be different levels of ability, meaning that you probably will get people who consistently win and consistently lose. This would mean that some people that rightly make up the lower end of Apex/Diamond would constantly be de-ranking, and this potentially could make queue times farrrr longer than anyone would like.
It might be an idea that once you get to Apex the mechanism for going DOWN in ranks changes to be slightly more forgiving. At this point instead of lost RP changing the bracket you're in when you cross the lowest part of the threshold : make it so that a certain number of losses outside of the top 10 squads means you de-rank. I'd also allow Apex lobbies to be populated with Diamond players if the queue time is predicted to be longer than 5 minutes.
Ranking and bragging rights are all well and good but ultimately the real motivation to KEEP playing ranked isn't to achieve the rank you're going to stay at : it's to get higher quality, more difficult games by being matched against players roughly your skill. If the ranking system ends up meaning that you can't get these types of games because the queue times are too long: people will have to resort to going back to trying to queue up multiple teams at the same time to manufacture harder lobbies.