Ok, for those who are saying 'a tall empire doesn't reflect history well enough' or 'lots of cities should always be best because it is realistic'
This is a game. It is not reality. There are many things in Civ (and all games) that are not realistic. Moving units around on a mechanical hex based grid is not realistic, but games are made up of many mechanics. In this case, those mechanics, I hope, are attempts to make a fun Strategy game at its core. So, first we have to answer what 'fun strategy' is...
I think that most people would agree that in Civ, 'fun' would involve a diverse range of strategic options. It is possible that I am wrong here, and perhaps strategic diversity isn't important at all, maybe things like immersion/pace/theme/controls/sound etc are more important. But I think we can all agree that Strategy is at least 'up there'.
If this is the case, then imo Civ6 struggles to achieve the same Strategic Diversity that other previous strategy games have offered in the past. To me Strategic Diversity is about having options, different ways to approach the game with grand strategy in mind. Now Civ6 does have lots of small tactical equations to solve, building small combos (think adjacency bonuses coupled with 100% adjacency policy cards) Where should I place this district to maximise my yields? What shall I prioritise building to maximise that eureka boost? This is enough to carry the game for some people, but for others we need more grand strategy options, simple tactics is not enough.
Wide vs tall is just one aspect of this, but it is an important player in this strategic depth discussion. I agree with the OP that it isn't present here, and I wish it was.
I have written another thread about all of this that got moved to the 'ideas and suggestions' forum:
https://forums.civfanatics.com/threads/improving-civ6s-strategic-depth.650084/#post-15554355