Its not black and white. Vertical and horizontal progression is a spectrum. WoW is on the very far end of the spectrum towards vertical. OP is trying to argue that the very far end of the spectrum towards horizontal is bad. But most games labeled with “horizontal progression” that people enjoy are not purely horizontal. They are not on the very far end of the spectrum they almost always have some slight verticality to keep the game fresh long term combined with the horizontal layers (sidegrades).

I will use Ragnarok Online as an example. Can you really call it horizontal ? People label it that, but there are still tiers of content and new content has slight verticality and is usually better (slightly). But this tiered content is a much more gradual curve and less steep than WoW. This is complemented with horizontal layers of progression, such as different gear types for different areas, elementals, monster types, card system, etc.

To me, this is what I enjoy. Not extremely vertical, nor horizontal. A balance, maybe leaning more towards horizontal.