Making it harder to acquire is meaningless. It'd still be the same issue of whoever went Nobles and got lucky enough to get Kayle auto-wins against everyone else except other Nobles and Void-Sorcs.
It's already hard to get Nobles cuz of Kayle, so that's not the issue. The issue is once they get it it's just too damn strong. In this game where adaptability is flaunted as a skill you need to climb we have Nobles just throwing that out the window and winning with dumb luck unit drops.
Here's the direct reasons Nobles are stronger now:
- Nobles were given MR in compensation for less Armor
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- Nobles then got both values buffed by 10
- Morello and Red Buff both got nerfed
- Cho'Gath got nerfed (pretty much the anti-Guardian w/ Void), which is why Sorc is a necessity
- Knights buffed which synergizes heavily with Guardian and Noble
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- Guardian buffed which synergizes heavily with Knight and Noble
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- Knights are also the uncontested best early game class right now and the Guardian buff is covering for mid game, smoothening out Noble's power curve
- Individual Noble units got their Attack Speed buffed specifically to synergize better with Noble
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- Fiora isn't a trash unit anymore
- Overall player damage taken is reduced now, meaning people tunneling on Nobles won't die out as fast (and with the above buffs, they usually won't be dying early at all)
There's also a bunch of indirect reasons such as Cho being nerfed (core unit of Void's ridiculous rampage last patch which destroyed Nobles), Volibear nerf, the introduction of Hextech which doesn't really bother Nobles as much as others, etc.
I'm not saying any of the above were bad decisions. Each individually was a good balance decision (except the direct Noble buff) but they overall led to the massive powercreep of Nobles.
The entire reason Noble got buffed was because they were underperforming last patch, but they were underperforming because Void Sorc was absolutely disgusting and unbeatable by Nobles.