Often it's cheaper in terms of marginal cost in dollars to buy new, but this does not factor in the environmental cost of having built two devices and disposed of one, or the opportunity cost of unused devices.
In the same way that initially Linux enabled users to repuporse old pcs and eventually that tech made it to revolutionize many other areas. I get the feeling that, if right to repair bills had teeth, we would see similar innovation phones, vehicles and other consumer devices.
There is so much underused existing hardware that only a change of law and a few small projects (a consumer/hacker/small business friendly re-installer) could have significant environmental benefit with close to zero environmental cost.
Phones are amazing tech these days but they get thrown away at the same time newer/larger/less powerful single use devices (Alexa/music players/home automation) are purchased new.
Economies rarely measure wealth (because its hard) and target income and profit. For humanity and the planet, wealth and efficient use of that wealth is what matters.
Two clear examples.
Software to re-purpose an unsupported phone into a iot device, voice activated device, media player or back into a working & secure phone.
Software to turn an unsupported self-driving vehicle back into a manually driven vehicle or vehicle with driver assistance.
I think the software required is easily within reach in a five year time frame.