HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

Although, in my opinion, it's still worth considering allowing the AI player to use magic and have morale in Quick Battles.

The AI player already uses magic in Quick Battles in HD+. What does it mean to "provide for morale"? Start a battle in manual mode, uncheck the spells in the auto-battle options, press auto-battle, and you will see exactly what happens in Quick Battles.

Introducing new units and objects does not mean changing the gameplay and spirit. The main thing is to preserve the atmosphere, which, as I understand it, the team working on HotA is currently doing.

It depends. For me, it definitely has an impact. The same Dock and the redesigned old creatures forced me to uninstall the mod and reinstall the game. Even the somewhat ambiguous Conflux and other innovations, in my opinion, don't cause me such a strong rejection.
I'm sure that if HotA had been released 10 years ago by NWC-3DO, you wouldn't be talking about any kind of incongruity here. The quality of the work of the 3D/2D artists on the HotA team is at least no worse than that of the original developers. And if the Dock had been released before the Conflux, the inadequacy of the Conflux idea would have been even more noticeable.
It's a pity that conservatism and years of habit won't allow many to enjoy HotA.
Of course, everyone has different ideas about the Ideal Heroes, and you can't please everyone. But does HotA claim to be the Ideal Heroes? "Unfamiliar" does not equal "bad." And the redesigned original creatures (I, for example, don't like this either) are minor things that you get used to and that fade in comparison to all the positive qualities.
From an aesthetic point of view, no gameplay mod for the third game comes close to HotA.

And in general, if adding a new city automatically spoils the game, why download HotA?

HD, by the way, doesn't please everyone either, and the fact that these new elements and changes to the original are optional does not redeem HD in the eyes of staunch conservatives.
Although, in reality, I think that the problem is that HD simply does not correspond to THEIR ideas about the Ideal Heroes.