Not following you here. Once one company went to one list, the others (assuming you're referring to regionals) would follow or wouldn't get new pilots? Sure they would. The WO aren't big enough to support every regional pilot. And regionals can't dictate to mainline that they must be included in the seniority list. Plus in many cases it is faster to get to mainline outside of a flow. Why wait 6 - 10 years to flow when you can get there in 2-5?

Speaking of mainline, which is where nearly everyone wants to go (not regionals for life with some exceptions)... once one company went to one list, the only pilots they would be able to get would be from their WO. Why would anyone from outside the flow go to that particular airline?

Let's take AA for example. If they went to one list with legacy seniority number from day one of Envoy, or even as a cadet, no military pilot would go there. No outside regional pilot would go. No 91/135 pilot would go. No LCC pilot would go. Why? Because they would be at the bottom of the seniority list until every flow has passed them, for the next 10ish years!

Eric signs with Envoy, not AA, Jan 1st, 2020. Mark the Military pilot signs with AA, not Envoy, Jan 2nd, 2020. Using 10 years just as an example to flow, or 8 or whatever, every single flow ahead of Johnny will be ahead of Mark when they get to AA, despite Mark having been at AA the entire time. Nobody will go there outside of flow because they will be at the bottom of the list for a decade despite thousands of pilots coming in.

The rest of mainline without WO will get experienced pilots from everywhere - 91, all 121s, 135, military, and even steal some regional pilots from the WOs. If there is ever an issue at the WO, it puts the parent company in a very bad position because they won't be able to recruit from outside.