Agreed. To some extent, what bothers me isn't just the lower mana on mages (though that's certainly an issue), but the fact that their mana was lowered without major changes to their spell costs. At the end of the day, mana, regen and costs are all based on each other, as well as itemization: for example, a champion with only 100 max mana and 1 mana regen, who consumed 10 mana per spell cast, would be no different in a vacuum from the same champion with 1000 max mana, 10 mana regen, and a 100 mana cost per cast. The difference is, if you have an item in the game that gives +100 max mana, it's phenomenal on the first champion, who gets to double their mana pool, but not all that meaningful on the second, who only gets a 10% increase. I can understand the gameplay reasoning behind reducing the mana pool of mages, if the intent is to make mana purchases more impactful on them, but I still think it's a complete thematic mismatch, and I absolutely hate the idea of mages all being pushed towards itemizing for mana. Moreover, if the intent is to push them towards mana itemization and to nerf their waveclear, then their non-clear abilities should be having their cost reduced: the fact that this hasn't been done suggests that Riot isn't being entirely honest on why they're nerfing mage mana, or simply don't want to make more detailed changes to their mana consumption, aside from reductions to Lux's E costs (which itself defeats the whole purpose of trying to nerf her own notoriously excessive waveclear, and screams of favoritism).
I very much agree that the philosophy of giving champions a stat deficiency, just to create a need to itemize for that stat, is a terrible idea. There's a critical difference between synergizing with a stat and requiring said stat, and while I think the former is good (your champion goes from feeling good to feeling awesome), the latter feels really bad, because until you buy that stat, your champion's gonna feel like shit due to how dysfunctional they are. This is why I especially hate the whole notion of mana problems on champions, because if a champion needs mana items to function, that means their core kit cannot function properly on its own, and that's a crappy feeling. In the end, mana is purchased not because it enables awesome gameplay, but because it makes a problem go away, which I think is the absolute least fun and interesting kind of customization to have.
To go back to the subject of Ahri, which we've talked about a few times before, this is why I think she deserves to not have mana issues, including in lane, because ultimately Ahri as a champion is meant to exude smoothness and freedom, including in lane where she should get to have a good measure of control and agency. Clipping her spell usage by having her chug too quickly through her mana doesn't really feel great for the Ahri player, and I think is a poor balancing lever for whichever excessive safety her kit used to have. I don't know how you feel about her current kit, but it feels like she's been given more interactivity and skill expression, which in turn allowed her W mana cost to be less ridiculous. These mana changes are likely to give her a bit more freedom in lane, but I feel it's going to be annoying as hell if she finds herself forced to run a mana item later on just to be able to do her thing.