Recommendation on Fixing the Inspiration Tree: Buff the Others
In the past, it would have been odd seeing a rengar take the inspiration tree as even a secondary, but now it seems like a given. But how is it that even assassins benefit from a tree designed to be more utility focused, and not damage? The reason is clear: the inspiration tree just feels better than the competition.
What I mean by this is that, when you pick the inspiration tree, you see immediate results: perhaps stopwatch allows you to get first blood; or biscuit makes an oppressive lane not so oppressive; or that the fluidity of summoner spells makes you feel in control of the dynamics of the game. This isn't in the other trees, unfortunately.
The inspiration tree is not overpowered, but rather, the most well-designed tree in the game. The real problem is that the other trees lack the satisfaction that the inspiration tree provides. As such, I believe that if the other trees were to be buffed, with a focus on making them feel comforting like the inspiration tree, balance will be achieved; while nerfing the inspiration tree will just make the current rune system feel as lackluster as the previous, setting us back to square-one in terms of progress.
Looking at the precision tree, we see the problem that, although the added stats are helpful for auto-attackers via dps, it does not solve the problems most face; for most adcs, that problem is kiting. When tier-3 boots were removed, many adc mains were caught off guard by how hard it became to kite once more, as many had been relying on the enchantment 'furor' that gave them 12% movement speed that decays over 2 seconds. This enchantment was not 'good', in the sense that it allowed adcs to burst more easily or just do more damage, but rather it felt satisfying. This is the issue that the precision tree has, as the only real benefits it gives are to dealing damage. For this tree to be fixed, it needs to fix the mobility creep issue adcs face.
The Domination Tree's problem is that is does not really 'dominate'. Instead, it gives some damage, sustain, and vision control. Although assassins like damage, as it helps in bursting down squishies, the thing that would most truly benefit assassins is the duskblade passive that detects wards. This, alone, would surely draw people to the domination tree, as it would feel good to be able to roam without detection.
The sorcery tree, although comparatively useful compared to the other trees, still has issues of its own. While some abilities, like ultimate hat and transcendence, do a good job of feeling good for midlaners to use, gathering storm and absolute focus do not. Raw ability power is not very noticeable, as not many people really understand how much ability power is good; only that a lot is a lot. Focusing more on improving the midlaner's ability to roam and make flashy plays would make this tree more desirable.
Now finally looking at the resolve tree, we see the same issue that the precision tree faces: although statistically it is good, it just does not feel good. Now, there are good options that feel great like demolish, grasp of the undying, and aftershock, the others don't play out as well. The defensive part of this tree--iron skin, mirror shell, and conditioning--have the problem that their bonuses are not very noticeable. If these were converted into shields that add onto the health bar, and were to function like malphite's passive, more tanks would like picking this tree. Second-wind is also a failed opportunity, as the health gained from it is hard to notice; for this I recommend adding something like tahm kench's e effect to it, so that it is possible to see how powerful it is.
In ending this, the message I want to get across most powerfully is that nerfing the inspiration tree will have no long-term positive effect; all it will do is make this rune system as lackluster as the previous one. Instead, I think that the popularity of the inspiration tree should act as a lesson to how important making power noticeable is for these rune systems, and that adding that feature to the other rune trees will solve this problem quickly. There is also the matter of the benefits of buffing the rune system, which would make it so that decision making will feel more impactful, and that diversity in the meta will occur more. Singed, for example, went from unplayable in high elo to over powered only because of dark seal's potion-related passive that made it feel good to take on Singed. Likewise, it is only natural that the tree with the most feel-good items like stopwatch, biscuit, and unsealed spellbook would also be the most popular when there is no comparable choice. keep in mind that my suggestions for fixes were nothing more than examples, and clearly have flaws within them. Their point, however, is not to show how specifically to solve this issue, but rather to establish the tone of how to fix them: through making the other trees feel good, rather than being numerically better.
Riot, please don't take this away from us.