I find this curious. The concept of "should" implies that there is an objective reason for it to be that way. What would that objective reason be?

It is reasonable to assume that someone (or some group) at Delta evaluated the number of existing and potential members who would achieve each level, and the costs of supporting those members vs the income likely to be generated as a result of maintaining the program. It is likely that the jump from 2MM to 4MM was an intentional decision - that there was a *reason* that they didn't want to give Platinum to 3MM.

I realize that they are not as smart as us customers about running an airline, but what reason, other than "I would benefit from it" would there be for making that change?