It depends, tbh.
Its known that we do not have a very diverse game as of now, where 5-6 parts are top tier. A simple definite number of parts in a certain tier. Like the top 5 thingy... There might be 5 good parts, but a sixth part would be just as good. Still, it will have to go to the next tier.
But I see where you're coming from.

We might not always have the number of parts equal to the definite number we would have set for a tier. This might compel us to add lower tier parts on upper tiers.
If not a definite number, then at least the maximum limit of each tier, just like the one we learn in chemistry in electronic configuration. Not more than 2 electrons in the first shell; not more than 8 in the second shell ...
In a similar wat, not more than ___ in tier 1, and so on.
If a situation should arise that we have less parts than the maximum amount, it won't matter! We do not need to compulsorily fill up all tiers. For instance, should the limit of the first tier be three, we CAN have only one, or two parts in it. But we can't have more than three parts there..
What'll be the benefit is what you may ask, but the benefit is something that I am unable to explain ...

Somewhat like standardization, which would avoid too many parts from being in a single tier. You see, Basalt and Death in the same tier would lead me to believe that Hell performs equally well as Earth, which isn't true ... Standardization would probably force us to push Earth in the next tier and all...
I know it might as well be the most terrible idea one would have heard of, but yeah, I just wished to have some opinions on this.
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