I'm sorry but I lost you here; Why would the current seniority system be unfair for DEP?
I don't thing there's anything 'unfair' to DEPs. It's more that if you join as a 30+ DEP, you'll never have enough seniority to be able to do anything 'useful'. Looking in from outside, it looks more and more like joining much older than mid-twenties may well not pay off (depending on what you want from your career of course).