Do you have a calculation for that? It's not entirely clear that this is the case if you factor in that the old hardware already exists and new hardware needs to be produced.
You're also looking at it from the perspective of not needing new hardware, but I already have both my phone and my Steam Deck.
Incidentally, I bought a Steam Deck in part because it was cheaper than a GPU upgrade for my desktop PC, while being more powerful and efficient than my current setup. My PC has been collecting dust as I boot the same disk on my Steam Deck, at 1/10-30 the power enveloppe. As an added benefit, I don't have to turn this high-power system on while it's 35C in the room.
The actual math of concrete examples we can come up will not change the underlying fact that consumerism fueled by corporations will always trump over whatever perceived or real efficiency gains, and i do not agree it ''pays back relatively quickly'': even the favourable example of replacing an Xbox Original for a Steam Deck demanded at least 2000 hours for the electrical consumption to match the incorporated energy, but this still had to incorporate whatever is needed to reprocess the xbox as the new trash it became (recycling and discarting). What will the math look like in the future ? because over time, the tinier processors demand more and more energy and resources to manufacture, over the older ones. How many hours of use will the (presumably) more eletricity efficient PS6 demand to offset the PS5 it will be set to replace ?
It does not matter if the Steam Deck, which is subsidized by Steam like almost all consoles and because of that it is cheaper for the end-consumer, consumes less electricity, it still made you pretty much discard a fully functional and capable computer and buy another one, a computer that will degrade with time and have less lifespan, that could have continued in use with a technically possible upgrade.
I feel a computing culture that sees normal to discard functional computers every 3-5 years because of minutiae like slightly less eletricity consumption or even more facial hair in characters will never really be sustainable. Eletricity consumption is being solved with renewable energy, hidro and nuclear , but the rate of material and natural resources consumption, and pollution, of non renowable stuff, is not being tackled.