Since CK3 is about grand strategy, not a card game aimed at “multiplication,” should the devs put a cap on 'Knight Effectiveness' and 'Prowess'?
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Why? Seems that you just highlighted a good example of a long-term generational strategy paying off.
There is a reason the example you are pointing to is a battle fought in 1240 from a campaign that started in 867, and that's because the variety of bonuses accumulated
would not be possible in the later start date. Gathering that amount of renown for legacies, artifacts with the right bonuses, eugenics successes, and so on is literally a multi-generational effort- in this case, over 350 years.
By contrast, most player campaigns are lucky to reach 100 years...
because that alone is enough to break the power curve. Not on the basis of knight effectiveness, but the general difference between player ability to power-scale versus AI. This is a general point for basically any Paradox-style strategy game: if you can win in your first 100 years, the later-game is just a matter of finishing up.
So your problem is something that only occurs if a player spends over triple the average campaign time, single-mindedly pursuing a specific goal, well past the point that AI competitiveness would be dead in any given paradox game.
In so much that this is a problem, it is a self-inflicted problem, and in so much that this is a benefit, it is a benefit from playing to the game's strategies for compounding benefits beyond the point of intended strategic competitiveness, which is a general limit in most strategy games.
Moreover, even if you want to say space marine knights is a problem, you're conflating the symptom and the cause. Knights aren't OP because late-game knight effectiveness is uncapped... knights are OP because
knight academies are broken in the earlier game. Knight effectiveness is nice, but far more relevant for the competitive phase of the game (the opening centuries of an 867 campaign) isn't the knights, but the +1 MAA stack modifier. Which allows bigger MAA stacks, including siege stacks, which allows easier and faster wars for accelerated blobbing.