Market demand is not the only factor, though. Manufacturers make design decisions based on a variety of factors, from supportability and manufacturing efficiency to alternative profit vectors like bloatware and proprietary ports.

If someone made a slider phone with a physical keyboard, it could be the best selling phone on the market without making the most money for the company.

Technically true, and niche devices with QWERTY keyboard like the ones from PlanetCom still exist. But they don't really benefit from economies of scale, are prohibitively expensive, and are usually at least a generation behind in hardware.

Plus Apple started, and Samsung joined, the "thinness wars" that got us to where we are today. Slide out keyboards were definitely a casualty of that, and I still hold some hope, albeit slim, that those could still make a comeback.

There is demand though, it's just not as high. They could make a smaller number of them just to capture the people who want it. Same goes for all the other features that are hard to find on a phone anymore. I think a lot of people are confusing "lack of demand" for "the features they want aren't available so they just buy whatever the corporations are jamming down their throat when they need a new phone". I for one haven't purchased a new phone since 2016 because there's no option that has more features than my current one. If it were to break I would be forced to buy a new shittier phone that can't do everything I want.