Yeah, horizontal progression for sure has strengths and weaknesses. But I don't think I'd worry about eventually everyone becoming 60. GW2 has had horizontal progression for fifteen years and their starting zones are still filled with new player characters. If done well, horizontal progression doesn't get stale.

Genshin has a good model here. Level cap has been the same since day 1 (6 years) and the increasing difficulty comes from new types of challenges. WoW would obviously work differently, but you do gear progression until you can clear endgame, then new regions introduce new boss mechanics that you need different gear for. The old gear at the highest tier and excellent gameplay is enough to clear for like 3 years, but eventually you get powercrept and have to invest in new stuff.