There is already one incentive. When you move to the next age, losing all of the adjacency bonuses often results in some of the second- and third-tier cities falling into negative happiness baselines due to the lingering happiness maintenance cost. I've learned to hold off on a few late-era buildings that I don't need in order to avoid this obstacle, while making sure I add the villa, menagerie, etc. in order to boost my baseline happiness for the next age. Plus, it can be helpful to preserve some food and production tiles as long as possible, along with maintaining reserve space for late-game wonders or specialty districts.