It's a bit too soon to say that MRI scanning is "harmless." Better to say that harm has not yet been proven, as an MRI is a relatively novel test.
An MRI of the head takes about 45 min in the machine.
An MRI of the entire body would take several hours per person.
300 million people x 3 hours each = 900,000,000 hours.
That's 1.3 million months for one pan-scan of the entire US.
Not counting the time it takes to look at all those pictures.
Even spread among multiple MRI machines, that's nuts.
Every 2 or 3 months as you suggest.....would be inane.
Also, what if you find a thyroid nodule or an adrenal nodule or a liver cyst? All three of these are VERY common, and mostly benign.
Are you going to sign all of those people up for biopsies now? Those aren't without risk.
I'm glad you left CT scans out of this, as they do have ionizing radiation and have the potential to cause cancer if you did them enough.
An ultrasound isn't a great tool for picking up small asymptomatic cancers.
You don't get a great look at the pancreas, you can't see through bone to the brain, you can't see through air to look at the lung or the colon.
Also, many cancers don't have much better death rates even when diagnosed earlier.
It seems like survival time is longer, but it just seems that way because you picked it up earlier.
It's called lead-time bias.
Diagnosing anaplastic thyroid cancer is a pretty dismal diagnosis no matter what stage you pick it up in.
Same with pancreatic cancer.