Even after the bedrooms are built, the bathroom and kitchen serve until plumbing and fixtures go into the main house. Once the house is built, there is no value in removing the trailer, and it becomes guest quarters. With a proper roof over, it can last a long time.
But enough decades later, it is torn out to make space for a back deck.
Many of these can be seen along cross-country Amtrak routes.
This is the large majority of mobile homes I see and work on.
In terms of increasing the supple of affordable homes for people who are in suburban or semi-rural areas, mobile homes really are a mixed bag, for the reasons listed (which is then exacerbated by PE firms buying up parks and milking the residents dry).
Fast depreciation is an advantage to the used market buyer, since prior owners took the hit.
Unlike a permanent home which has mature ecosystems for completely refurbishing them, a better bet with a mobile home is to use it up and throw it away.
I've found that outside of light switches, electrical outlets, and bath tubs, much of the stuff is the same or similar (2x3 instead of 2x4 or 2x6 walls for example, or 1/4 foam instead of plywood sheathing) than regular houses, and can be purchased from Lowe's or Home Depot at regular prices instead of super high prices from the only mobile home parts store around here.
That is not true. There absolutely is a market for resale.
I see these mobile home land packages come up for sale on Redfin all the time. They do appear to appreciate in value if they are clean and taken care of, but not as much as a site built home. I have even seen some successful flips.
I guess the main drawback is that they are typically located on less desirable land and the lifespan of the house just isn't going to compare favorably to a site built house, but at least they are affordable.