it means nothing to higher tiers. tiering megas separately does not impact them in even the slightest way - the same pokemon would still be used, the same megas would still be just as effective, the metagame would evolve in the same way. tiering megas separately only impacts lower tiers, which simply get to have more pokemon.
to be more specific and to use an example: beedrill-mega is UU. beedrill-mega is a great pokemon, with very high speed (145) and attack (150) and adaptability (the ability, not the descriptor). regular beedrill, on the other hand, is a terrible pokemon, with stats that are half as good and is very much impossible to use effectively in any environment other than PU. without separating megas, beedrill and beedrill-mega are both considered UU. with separating megas, beedrill-mega is UU but beedrill itself is PU, allowing PU to have more pokemon available but not changing UU whatsoever (since UU wouldn't have used beedrill anyway).
so no, leaving tiers as they were is generally considered less appealing to the lower tiers. not everybody agrees on this, but the disagreements are not because of people who misunderstand the purpose of tiers (you can use pokemon from any tier below the one you're playing in!). some pokemon may be undervalued in higher tiers, but it is not "better" or "worse" to not play with them.
i suppose the point i am trying to make is that encouraging people to use lower tiered mons does not directly correlate to "diversity"; the best mons will always be the best mons and will continue to see more usage than the ones that are just OK. one of those lower tiered guys will eventually be seen as great (best example in recent memory is weavile), and it'll jump to OU like the rest. it's the life cycle of tiering.