I don't have a strong opinion on which is a better game, civ IV or CiV. I just feel it is very clear that CiV has been more successful so it seems unlikely that they will try to revert much back to the way it was in IV. I do agree that more expansion than V would be nice. Personally I would enjoy nearly endless expansion and have the only limiter really be the map. That obviously won't be the case in VI and even though we haven't seen it happens, everything that we know about the game rules leads me to believe the ideal number of cities in VI will be greater than that of V even if not by a ton.
1) All the cost of cities (as far as we have seen) is up front. Settler costs increase, district costs increase... but once you complete these costs the cities always add maximum yields. This means having another city always increases your output and never hinders it. The only real cost is the opportunity cost. This is better than CiV.
2) Cities having overlapping tiles is actually beneficial. All districts get adjacency bonuses for other districts, and I would expect being next to a good mountain range or river is more important than working every single tile (With housing and amenities working the way they do I expect population totals to be lower end game... like they were pre-expansions in CiV). So if you have less pop, you need less good tiles to work, so good adjacency becomes more important... potentially. Also there are districts that effect every city within a certain range (Electronics factory). If cities are clumped closer together, and costs are upfront, you will see more cities in a smaller amount of area.