This is disingenuous.
If price was the reason, we'd have phones with longer term support at a higher price point. The problem is that support is cheaper than making new stuff, but the companies would rather sell a more expensive good if they can prevent customers from buying the cheaper option.
Support is cheaper for customers because companies providing support have to invest into something customers want, but doesn't create the same competitive advantage in the market where stakes are always rising. I.e. companies are incentivized to replace rather than support because they need to amortize the cost of R&D that goes into making phones with more memory / storage / cameras / hoo-hoo-ga-ga. If instead the effort is diverted into support, that effort doesn't generate future revenues as much as the effort spent on, well, future technologies.
So, the customers don't get good service not because it's impossible or prohibitively expensive, but because of the lack of a free platform (equivalent to Linux on PC), and a lack of regulation that would make it necessary for manufacturers to provide longer term support to their products (which would've leveled the field, because everyone would have to provide similar length of service, and so the competitive edge would stay the same).
As long as the manufacturers are in the rats race for the fraction of the future market, they'll cut every corner possible to get a bigger slice. The cost and the price today aren't as much of a concern for them as the survival in the next few years.