I wonder if a city could just decide to turn all the land titles that detached homes sit on, from freehold titles, to life-estate titles.

So, it'd be like eminent domain, but you can keep living in the property (if you do live in the property) until you die. But, if you were to try to sell the land, you'd only be allowed to sell it to the government, at the market rate (as they would pay in under eminent domain); where the city would then tear down the detached home and (presumably) re-zone the land for highrises or mixed-use buildings.