I’d like to start by saying: Will anything but 121 experience give you 121 experience? Not exactly. But you must have a good base to begin with in order to properly and successfully build the 121 experience.
Just cuz someone has 1600 hours doesn’t tell the whole story. Rhino Driver made a good point. In addition, I’ll add that I still consider someone with 300 hours coming in the front door and the rest of the time accrued in a year and a half to be inexperienced, period. It is very rare to find a job other than the regionals where you can “drink from the flight-time-building fire hose” like that. And most of those jobs don’t involve taking 50 to 100 people with you on each flight. Part of “experience”—especially at the beginning of the learning curve—is accruing that flight time at a slower rate so that experiences, thought about what is actually happening, working on ways to improve your skills, etc. etc. have time to sink in (even while working as a lowly flight instructor.) This is one of the major differences between past and present to me. We want to bypass a lot of the work and time that used to be required and “live the dream” NOW. Do some adapt to the “fire-hose” treatment quickly? A few. Ultimately almost all eventually adapt after a few more years of it . . . and rely on luck that nothing happens during those first few years that is too far out of “the norm.”
I think the ATP requirement is a great idea, not just for the check ride alone, but because it would cause people to have to slow down and take some time to develop. Rhino Driver also gave some other good reasons for raising the requirements that I will not re-hash. Would an ATP requirement automatically make every pilot safer? Probably not for “GOD’s gift to aviation” types. They will be good from the get-go. But for most of us folks with average ability (like me,) I believe it would make a difference. In the big picture, the overall odds would move toward the safer side. JMHO