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One of the reasons for said estimate is to test it's efficiency, which is being shown to be working well in the places that have been receiving it. The original idea behind only 12k was a compromise between advocates and opponents, with plans to increase to above 25K within a few years if it was shown to work, which it is.

Then what should minimum wage be if basic income is implemented at $25,000 USD per year?

I'm not a math Major lol. I just know that's what the original framework for the proposals was centered around and after compromises we got what we did, which I agree, is way less then is needed.
To be honest, the 15 an hour is even smaller then most places estimated minimum wage required to live above the poverty line. Each area has it's own estimate, but the average now is (based on gdp growth, rental costs and food of areas on average) up to $22/hr.
So, if I were negotiating the plan, I would have tried to go for at least 50k, which I think is still less then $22/hr... It's kind of a no win situation in the debate until people can see that it actually works which will take a while for people to get over their initial bias and propaganda feed.

Aside: There is a difference in how much it should be (for basic essentials, as is supposed to be the reason for it) vs what we want it to be (so we don't have to work for anything and can enjoy our own time). We're not far enough along in the process to be able to do option number two, but option number one should have been implemented years and years ago. It will take time, (and probably severe threat to the rich's wallets) to fully implement a system of UBI that takes away the need for work for compensation globally or even nationally.

Forgot to mention this too, https://www.mannabase.com/?ref=54b4c62029
manna.jpg There are lots of ideas, it just needs some support. Check this out if your interested. Cheers.