They aren’t sending the money to every taxpayer. There are income limits that will keep a large number of people from getting it. Fortunately or unfortunately that doesn’t effect me though I still haven’t gotten mine. Portal is still showing Not Available.
Overthinking the details
For a single person, the $1200 payment is phased out for an adjusted gross income between $75,000 and $99,000, reduced by $5 for each $100 over $75,000. The AGI is based on the 2019 tax return, or the 2018 return if the 2019 return hasn't yet been filed. I don't think we know when that determination is made. If it's at the time that the check is being prepared, i.e., not from the date that the stimulus bill was passed, then some taxpayers had a window of opportunity to file or not file their 2019 taxes, which would determine whether the formula was applied to their 2018 return or their 2019 return. Now suppose this was the case. Which return would they prefer?
For most people it won't make a difference. But if their AGI went up or down between those years, and either AGI is in the $75,000-$99,000 range, then they may benefit from using the tax return with the lower AGI. For most people, income gradually rises over their working career, then drops in retirement, so the first rule of thumb would be to rely on your 2018 return if you are working, but your 2019 return if you retired last year. But of course there are exceptions.
Example: John Doe got a raise that resulted in a 3% increase in his AGI between 2018 and 2019. If his 2018 AGI was $72,815 or less, it doesn't matter whether he files his 2019 return. If his 2018 AGI was between $72,816 and $98,999, and he has a choice, then he should not file his 2019 tax return until after the stimulus check has been drawn. If his 2018 AGI was $99,000 or higher, he won't get a stimulus check in either case.
Special circumstances like a change in tax status (e.g., after a marriage) can change the logic. A couple who married in 2019 and filed jointly for the first time might come out ahead with their separate 2018 returns, especially if one partner earned more than the other. And if a citizen was married to a non-citizen who obtained a green card and a Social Security Number in 2019, and they filed jointly for 2018, then they would get no stimulus check based on their 2018 return (because couples where one has no SSN are denied checks). But they would get a check based on their 2019 jointly-filed return. That's a difference of $2400 based on when they file their 2019 return!
In theory, evildoers with high incomes could have filed a 2019 return with a falsely low AGI (and perhaps some extra children), waited for their stimulus check, and then filed an amended return with their correct AGI. Would the IRS catch that? Would they end up convicted of tax fraud? Or would they get away with the crime of the century? Feel free to speculate.
Out of the hypothetical world, back in the real world
I hope that everyone who needs their check has gotten it or will have it very soon.