The place where I work is still using PCs from the late 1990s. They have been upgraded once, to add more memory. In this case, it's not the company's fetish with cutting costs regardless of the cost. It's a rational decision - those PCs are powerful enough to do the required work and replacing them with modern kit wouldn't be any improvement.
I don't think we're anywhere near peak tech, but in some situations we've reached peak usefulness in tech. The OP mentioned phones (which aren't phones any more) and I think that's a good example. For everyone I know, the limiting factor in their desired use of their phone is battery life. Everything else is more than good enough for what they want, so any improvements would be irrelevant. I also think that in many cases it's a temporary peak that will probably at some point be removed by further advances in technology. Imagine, for example, a "phone" (i.e. pocket computer with a phone app) that was also a fully fledged games console that ran the same games at the same performance as a modern console, had a battery life of days while doing so, wireless connection to controllers and TVs so you could easily dock it and use it as a games console, a projection display so you could have a large "screen" anywhere you had a flat surface and user input via gestures or a game controller that folded into something small enough to slip into a pocket. That would be useful enough to enough people to move peak usefulness in phones.