Steel is never going to vapourise at the commercial level. The day you see a small, sun-baked welder in Patagonia hauling TIG equipment from behind his ageing oven to repair your 8000 series frame is the day that happens.
90% of all engineering failures are fatigue based. I doubt you will ever have seen a pure tensile failure.
I've fatigued a bike till it broke. I'm six-six and 204lbs and ride like a maniac. Admittedly it went at the drive-side chainstay, so there will have been a SCC component to the failure, but fatigue is what killed the tube.
And as for the real, economic reasons for the material? Maybe to reclaim market share for Reynolds. Columbus tubing outdid everything they tried with 4130, En18, Nb+V-doped 4130, etc. Columbus don't have a DOD supplier based fifteen miles from their UK headquarters. Reynolds do...