Or farmland, because that's just unused space, right?

That's how it's been seen traditionally in the US, unfortunately. Silicon Valley in California used to be the site of the largest fruit producing area in the world from the beginning of the 20th century until the 1960s, but technology turned out to be more profitable. The Orlando area in Florida was also a major citrus producer until the 1970s and the coming of Disney World and other theme parks that located there to get traffic from Disney visitors.

What about the empty, smelly, marshland? Sure there may be some limited wildlife there but what about you know, people who want a house affordable enough to have a family?

Limited wildlife in wetlands? No. Wetlands are among the most richly bio diverse ecological niches. Humans regard themselves as somehow outside the system in which they evolved.

Smelly? The presence of volatile compounds interacting with your olfactory receptors that trigger certain aversive reactions is a poor justification for destroying a natural habitat.

The smell they're referring to is probably the Milpitas Stench, which is probably coming from a San Jose garbage dump. It is quite… noticeable.

https://milpitas-odor.info/about/

There are millions of square kilometers of undeveloped land that are not within 30 minutes of the most expensive real estate in the USA. Preserve it elsewhere, let families have homes here.