The only thing proved in these grounds is fairness is not a design input. The resources required for players who could not enter conquest and then at some point in their future make the 4m grade is not remotely the same as people who happened to be at a higher GP at the time conquest first ran for that unit. Think about people in 2-3 years time making it to 4m gp for the first time.
I have all conquest units , and see no reason to restrict them one year after release.. well apart from stoping cheating, which is why other events moved to a hard GP limit after people posted shots of legendary units unlocked with level one prerequisite characters.