What a defeatist thread to make.
Of course you can still "go tall", though I despise the distinction that has ingrained itself in the civ community between "tall" and "wide", since there's absolutely no reason why you shouldn't always have both big cities and many cities.
If all your cities are small, the vital trade routes that are supposed to ensure the growth of new cities will not provide enough resources, since the number of districts in the target city is what determines the trade route yields.
You will also not be able to place important districts unless you have enough population, and luxuries provide a check on expanding too much without building entertainment districts.
Civ V was restrictive and dull in how it forced you to only build a few cities, which isn't how an empire sim should be. Civ 6 is just back to how it used to be, a realistic sprawl of smaller towns around a core of large cities.