My personal take on this is that the problem with mid lane isn't simply waveclear, but the fact that waveclear has become the litmus test for mid in the first place. Assassins have been suffering not simply because they lack waveclear, but because neither them nor mages have been valued for their burst anymore, at least not at the competitive level. This is also why there is such a rift between pro and standard play: in solo queue, or any game with less organized teams, assassins are both present, and often quite dominant mid, because they still have the power to assassinate marksmen. However, in pro play, marksmen have such an excessive degree of protection that neither mages nor assassins can kill them properly. Because of this, mid laners are relegated to other duties, namely waveclear and CC/utility: since assassins tend to be hyper-specialized towards single-target burst, whereas mages have CC and AoE on the side, there is effectively little assassins reliably contribute in pro play, which is why they're not as present mid. As such, the supposed excessive waveclear of mages (which, by the way, is still behind the manaless waveclear of marksmen) feels like a scapegoat to avoid addressing the real issue, which is the excessive safety of marksmen in organized play. Because of this, mages are effectively getting punished for the sins of another class, and so in a manner that is unlikely to solve the actual issue with mid lane.
What also annoys me about the mana changes is that the implementation conflicts with the stated goal. If the intent is for mages to be less good at waveclearing, then the natural solution to that should be to directly nerf their waveclear abilities, either by increasing their cost, or by reducing their damage or safety against minion waves. However, when a champion gets the entirety of their mana nerfed, with no compensatory changes to their costs, they become less effective at casting all of their spells, including abilities that have nothing to do with waveclear. Mages are therefore getting nerfed across the board, even though they're supposedly only meant to have just one aspect of their playstyle toned down. It's also worth adding that Lux, perhaps the mage most infamous for her safe waveclear, and Ryze, a mage who's dominated pro play for years, are the only mages to receive changes to their costs that would make their waveclear easier, thereby putting their own changes in direct contradiction with Riot's stated goals.
Finally, my other problem with this set of changes, which others have mentioned, is that it precedes changes to bot lane. We're being told that mages are being nerfed because "no class should be dominant in a lane for years", yet marksmen have been the only truly viable option in bot lane, and have been mandatory since League started developing a competitive scene. We're getting serious, targeted nerfs to mages, but only vague, wishy-washy talk about "opening up bot lane" to more classes, with no concrete explanation on how that's going to be achieved. Moreover, if the update is to actually succeed, then it should target the overbearing amount of protection carries, and especially marksmen can receive, which should itself naturally bring assassins back to mid lane in pro play. Effectively, both the timing and the ordering to these mage nerfs are completely wrong, and are likely to age like milk the moment the bot lane update comes along, thereby leaving mages in a terrible state for much longer, when waiting until after the mid-season update could likely just be enough to address the reason for their nerfs.