I mostly agree with what you are saying. I think osu could have a meta, similar to how other games such as LoL and Dota 2. Both these games are fairly balanced at high levels, but what determines how popular certain champions/heroes are is how well they fit into the current meta.
The Meta is mainly determined by what strategies are popular (as to best preemptively pick/ban against what you believe your opponent will also pick/ban), as well as what characters the team's players have practiced. Natural meta evolution occurs when teams find ways to defeat the current popular strategy, which then becomes the new popular strategies and that eventually gets countered and so the cycle continues, possibly even coming full circle (This allows for every hero/champion to popular or viable at some stage). A Stagnant meta is when a single or a few strategies remain dominant for excessive amounts of time, in which the developer steps in. Forced meta evolution is when the developer nerfs/buffs/changes aspects of the game, causing certain strategies to objectively become weaker or stronger, hence creating a new meta.
Back to osu! The current meta heavily favors nomod and low levels (less than 50k), DT/nomod until about 5k, then HR becomes viable (mainly due to AR10 reading ability), with DT being the most dominant and the very top with a few (10 - 20%) HR players. Nomod at the highest level is competitively nonviable, simply due to the lack of maps, low OD (hence lack of pp) and incredibly difficult pattern complexity. HD is extra, anyone who can read HD usually just adds it to the mods they already enjoy playing. HD by itself usually doesn't give much more pp then nomod. Flashlight also gives no pp for how difficult it is, due to how it's hard to measure memorization as a skill, also reading with flashlight on is another less understood skill in itself.
Even if the pp algorithm was perfect and effectively rewarded each mod the perfect amount of pp DT would still by far be the most popular mod, simply due to the fact that it would still be the most practiced and most preferred mod, among the top players. Conclusion, if the developers would like to see the each mod be equally played at the top, they would likely have to under weight DT or overweight HR and nomod to give an incentive for current DT only players to also practice HR and nomod.
This causes a whole load of issues for players who already have a mastery in the less popular mods, Cookiezi for example. This may or may not change how many maps are created for certain difficulty levels. PP has been known to be a deciding factor for some mappers. If done right, this could very well make more diverse players/scoresflashlightplayersviablemaybe. If done wrong we could be screwed.