Don't know if that's the case, can see an argument either way.
For example it'd be unfair on a rider like Kemp who averages just over 4 & struggles to get a team, but Jenkins drops below 4, gets a guaranteed RS spot, then gets another big reduction.
But the downside of allowing it only once, is a young Brit gaining an artificially high average & missing out next year, not the plan behind the RS system.
I had previously thought, what if once a rider drops out of the RS system his average reduction goes down in stages? 1st year 20%, 2nd year 15%, then 10%, to a final 5% reduction. Raise the reduction for all Brits from 2.5% to 5%?