Talk to a typical owner of an old house, and you'll likely hear something about how they love its quirks. But would they build a house the same way today? Some probably would, but others would certainly not. This actually probably applies well outside of old homes too. I suspect something akin to the endowment effect (https://en.wikipedia.org/wiki/Endowment_effect) plays in here.

It also makes sense to me that someone would appreciate a historical landmark but place a lesser value on a modern replica of that landmark. Similarly, I could imagine a person admiring a historical neighborhood more than a modern recreation of it.

> Let's drill down and focus in particular on laws about multi-family dwellings. What logic would make them good when they already exist, but bad if you wanted to build more, and even bad if you wanted rebuild them after a natural disaster?

I'm not sure the answer needs to be logical (see above), but even if it must be, I don't know why placing limits on something implies that it was never actually good in the first place. Natural disasters are another story for sure, but I would imagine that sort of massive rebuilding was not the primary motivation for the present zoning laws in Somerville.