I don't think they're trying to make their old products worse over time. But I also don't think companies generally care that much about making old products work better over time. One of the parent commenters noted how well modern android runs on really old nexus phones if you strip out the "modern" animations and useless features. There's nothing stopping google doing this. People would love it. So its notable that they don't. Apple got a lot of good will from me a few years ago when they focussed on performance in ios 13 (or was it 12?). That OS release made my phone feel new again. After that update I think it ran faster than it did when I bought it.

Another way to think about it is that when you buy a product, your incentives and the company's incentives are aligned. You want the best phone. The company wants your money, and knows they need to deliver a good product to get it. After you've bought a product, the company's motivations aren't as well aligned with yours.

Arguably a company sells more phones in the long run when they have a good reputation for delivering on quality, and supporting their products. Eg, a few years ago some of my friends would buy every single blizzard game simply off the back of their reputation.

But most companies don't take advantage of this, and mistakenly focus on short term sales even if it harms their reputation. And, in turn, their long term profits.

As someone much wiser than me said, service and support is a form of marketing to repeat customers.