Is global minimum wage a good idea or not?
And why?
Nothing like minimum wages as they're applied within nations, no.
But if enough trading countries - poor ones, as well as the biggest economies - could be brought on board with the idea, certain restrictions on companies' privilege of trading internationally (ie, between all countries agreeing to it) might be a good idea: To prevent a race to the bottom, seeking the lowest possible labour and environmental standards, companies should be obliged to meet a certain fraction of those standards as they exist in the countries they sell to.
For example if you're a company producing and selling smart phones, most of your consumer base will be in wealthier countries but - to maximise profits - most of our resources and labour will come from poorer regions of the world. Your suppliers might get some of their minerals from the Democratic Republic of Congo, perhaps,
from militias who "have forced local people to dig in extremely dangerous conditions, have extorted minerals and money from self-employed miners, have tortured, mutilated and murdered those who don’t comply and have spread terror and violence – including gang rape and child abduction – through the rest of the population."
I see no very obvious problem with the countries which mostly consume such goods - with the co-operation of the poorer countries - simply telling companies that we don't want none of that **** happening in the supply chain if they want to sell their products here. And of course they do want to sell their products here, and bringing up the standards a little will rarely be a crushing blow to their business model (nor should we mourn if it's such an atrocious business that it would sink them!). Even a
fraction of the standards upheld in consumer countries will often be far better than would otherwise be the case. For example
India ranks 69th (out of nearly 200) countries for monthly average income of wage-earning individuals; yet if Indian employees of a company trading in the US were required to receive a mere quarter of the US $7.25 federal minimum wage, they'd be earning above average wages (even if they only worked 40 hours a week). In fact they'd be earning
far above average, because those figures for India are in terms of purchasing power, not the direct conversion rate.
Companies would still save plenty of money by using the cheaper labour in poorer countries, thereby stimulating those economies. But instead of the existing race to the bottom, and the perpetual fear for any country to unilaterally improve their standards because so many jobs might move elsewhere, we'd see the standards to which consumers in the rich world are accustomed
pulling up standards in the supplier countries (quite possibly with a follow-on effect even in local companies and industries which
aren't international traders).