The proposal cites Japan's

science bonuses as being a problem to be fixed as one of the reasons to do this change... And then adds a science bonus to the civ in place of one that is removed. proposal 24b resolves the specific issue cited with XP gain to my satisfaction, making this far more radical change unnecessary.
You got it wrong. I mentioned that Emperor AI Japan getting so much science from the leveling mechanic to the point that it can win SV without even trying to, far more than what a human or lower difficulties AI can, is a problem and an indication of an issue in the leveling mechanic. Japan having science bonus is fine, its AI at higher difficulties blowing it up exponentially with experience handicaps isn't.
This UA proposes to remove a unique thing that Japan does -- yields on units levelling up/gaining experience -- and replace it with a 2nd reward on a trigger that Japan already has: GGeneral/Admiral birth. This is a net removal of interactive elements from a civ. The civ is less with this change; it just does less.
I have nothing against the idea of elite units, or the leveling mechanic by itself. I just don't think the current experience handicaps allow for a healthy balance design with leveling mechanics right now. And we shouldn't be keeping an unhealthy design, as much as we may like it.
One reason I propose to move the yields from leveling to GG/GA birth is because both mechanics share similar fundamentals. Units generate both experience and GG/GA points in equal base amounts from combat; going into combat to earn one of them automatically results in earning the other as well (assuming a war against a major civ). The reason why Japan can have rewards for both GG/GA and leveling and have a cohesive design, despite neither of them interacting with the other, is because of that fundament. Both mechanics push you to do the same at their core: focus on units that generate the most points in combat (melee combatants). As such, the resulting playstyle from my proposal will share a lot of similarities with how Japan is currently played.
This condenses Japan's entire UA onto a massive reward for a single game event: 50% GWAM completion and 250


, scaling with era for GGeneral and GAdmiral birth. No other gameplay elements are in the UA anymore. The building yield bonuses aren't interesting, but at least they are a different kind of bonus than the 50% GWAM completion.
The early free Great General is actually an interactive element in itself.
Remember than Japan lacks early militaristic boosts, in contrast to most militaristic civs, due to its "fight stronger when wounded" mechanic being moved to the Dojo years ago. The free Great General gives Japan a pre-Medieval military effect, long before before other civs usually can get a GG themselves, and is flexible enough for both barbarian hunts and early wars. Notably, it makes the Classical Era Swordman rush strat more reasonable, as you can count on a +15%

CS aura quite before your opponent can. And, by the time both sides generate a Great General naturally, you have a spare one for a citadel if needed. Last, if you are attacked before Medieval, that free Great General helps dealing with whatever combat bonuses or UU your aggressor is bringing against you.
All of that is far more interactive than the current "


yields on defense/military buildings" mechanic, nevermind the nerfed version on your proposal. And that free Great General actually interacts with the rest of the UA, unlike the defense/military buildings mechanic; Japan's overall design ends being more cohesive.
If Japan didn't feel locked to Authority/Imperialism before, they are with this. With GGeneral faith purchases unlocked in Industrial, you can get 50% progress towards 3

GPeople types and 1000

/

every 5 turns for

Faith purchasing GGenerals and Admirals. Japan's kit would now revolve around these two faith purchases as core to their late game, to the exclusion of any other concerns or alternative policy paths.
To the Glory of God exists. And you can only buy so many of these two before the faith costs become prohibitive, so being able to buy other types (such as Great Writers, which Progress unlocks) is a valuable asset regardless of which path you take.
Note that faith purchasing Generals and Admirals isn't something you normally want to do early with Japan, as the 50% GWAM progress can put the cost of the next GWAM way too high for your specialists in the Capital to fill, especially if you do like you said. You can generate more

GWAM in a game by faith purchasing Writers/Artists/Musicians first, and leave the General/Admiral purchases for later, when the faith cost of the cultural ones is prohibitively high.