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In terms of ideas, the fourth installment can definitely be called a breakthrough. As for the fifth game, it was a project that would have been difficult to ruin. Firstly, there was a significant time difference between the releases, and secondly, a huge number of ideas for improving and developing the strategic elements of the game were gathered from the third and earlier installments (I also consider the third game to be essentially an expansion of the first and second parts).
Therefore, the developers of the fifth game essentially had the task of more or less carefully collecting the ideas from the earlier versions and somehow combining them. The result was controversial, but that's okay. What's bad is that it turned out to be, in my opinion, simplified, although this is quite understandable given the need to appeal to a wider audience.
On the other hand, I highly doubt that a "complex" fifth game would have been successful in the gaming world, because the new generation doesn't really like to learn complex games, and the old generation already has their favorite installments. So, this is perhaps a double-edged sword.
As for the sixth installment, it's difficult to predict what will happen. If it's just an improvement of the fifth game, then the chance of the game failing in terms of interest is probably 80-90%. The sixth installment needs serious development from the previous parts, that's one thing, and two, the game needs to have balance (perhaps the only thing that the fifth game can be praised for, if you ignore the number of patches :D).
In general, I highly doubt that the sixth game will be interesting.
All of this is just my opinion and is based on my personal (albeit not very extensive) experience playing the games and reading various articles and posts