But as I understand it, the AI level will also be lowered. As a result, it will be uninteresting to play on medium, and impossible on hard.

The AI level will definitely not be lowered; only the set of tools for the player to defeat it will be expanded.

I don't think that switching from normal to hard will be a big problem for a more or less experienced player.

Added 2 minutes later

I see neutrals as an additional obstacle. You – as the main opponent. Eliminating neutrals, even very quickly, will soon become boring. It's monotonous. For a good offline game, interaction with a good AI is needed.

I agree, a good AI is even more important in a strategy game. But they didn't provide it, and whether such an AI will be implemented in Eternal is unknown. Therefore, the option with neutrals is a backup. It is a very simple to implement and, at the same time, effective tool (although not as good as AI).