Nobody "gives up a spot". Any new corps which makes it into the top 5 has to earn it by being better than one of those corps... same thing for each additional rung of the placement ladder on up.
Every corps every year tries its hardest to make the best show it possibly can, so BD, Cavies, Cadets and anybody else will continue to do that. What I think will eventually make the difference is which corps can retain its outstanding design and instructional staffs (or even improve them), and which corps has the most veteran membership.
Having a strong, veteran group of members can not be overstated here. It is key to being a cohesive and focused group.
Invariably, though, someone each year will lose a lot of veterans to aging out and/or defection. Whomever is best able to avoid this problem and continue to restock their talent pools will be the ones who continue to flourish. (I really think the staff and talent turnover issues both hit SCV at the same time in a big way a couple of years ago. They're just recovering now).
These things happen to every corps to some degree. It is impossible to avoid completely, but it can be offset by having a very strong talent pool to pull from (like BD & Cadets) when they need to restock, and by having the great designers pass on their tricks of the trade to their replacements (like Cavies and Star) when they are about to lose a designer.
I love this aspect of the activity, and it reminds me a great deal of the issues that college sports programs have because their athletes are limited to only 4 years of elligibility maximum. It keeps the game from being completely monopolized by one or two groups entirely, and allows opportunities to others.