There are economic and environmental costs to everyone having the tools, skills and supplies to repair a reasonable range of products. At the least it likely makes more sense to have a specialist perform any but the most basic repairs rather than doing it yourself.
In some cases repairs make sense from an efficiency perspective, in some they don't. All I'm really saying is that it's better to evaluate the more efficient choice dispassionately rather than painting repair as somehow morally superior to buying new.
Buying new is often not optimal since the stuff you buy istantly is worth something like 20% less from the moment you buy it.
You pay to avoid the risk of buying an used object, for example, but I'm not sure that buying new is the more efficient choice every time.
Buying new may or may not be optimal in any given situation but the resale value is only relevant if you decide to resell, it's not a general argument against buying new.
It's almost certainly not economically efficient for a software engineer
to be darning their own socks.
I don't know about you, but I don't do a very good job of software engineering 100% of my waking time, even if doing so would certainly be more economically efficient than the alternative. 8 hours a day? Sure. 10 hours? Maybe. 12? Probably not, especially for extended periods.
We all make decisions about how we spend our time, both on and off the clock. Personally, I'm perfectly capable of sewing a loose button while watching TV and still consider it downtime. Even if I could easily afford to buy a new shirt instead.
I agree with you that you shouldn't at all costs repair everything or do everything yourself. But I'll happily assign some of my spare time to maintenance tasks, and I think doing so does make economic sense as well as environmental.
Environmental economic impact are usually only felt when the damage is so severe it starts to affect the supply chain while a company is still in business, which may take years or decades to manifest. By then the economic cost of the environmental damage may be too great for the company to properly deal with. All too often companies leave behind environmental devastation for others to clean up, and are absolved of the actual economic and environmental harm of their actions. Perhaps it becomes something that society has to deal with through higher taxes, but that just hides the cost rather than including it in the price seen on store shelves.