Depends! Increased density should drive up the price of land.
However, if you are in a neighborhood of single family homes and a huge multi family building is constructed on your block, your home price will go down because there will be enough supply to meet demand, taking pressure off the single family homes.
If you are in an already dense city and regulations are relaxed so that more high rises can be built in more neighborhoods, the price of your condo all else being equal will be lower because there will be more supply. The land price may well go up but your individual apartment would likely be worth less.
a huge multi family building is constructed on your block
which is why NIMBYs should at least be in favor of gentle density - duplexes and triplexes, small walk-ups, etc. But they fight those like the end-of-days too.
And then you're back to the original argument: the appreciation of your neighbor's property isn't some kind of god-given right, no more than that of yours is.
Look, I don't want to destroy the value of the largest "asset"[0] most people will own. But... what's the alternative here? The incumbents just get to grow fat off riches that they absolutely lucked into and played minimal part in creating, while lower-income residents get forced out of their homes? We just give up on the bay area as a worthwhile economy, have all the tech companies pull out, and then home prices crash?
[0] People really really love to believe their primary residence is an asset/investment, because it cost them so much money to acquire it, that it just has to be. If they'd consider it just a cost of living, a liability even, we'd have such fewer financial issues in housing markets.
Doesn't increased density help property values? If I can build a high rise on your 1 acre plot of land, its value should go up.