As a barely employed CFI not even considering the regionals for several years, if they started requiring ATP to fly FO (which when I got into aviation, I thought they did... my mistake then), at least to me and my fellow instructors at my school it's just another checkride.
We think of it this way: most airlines, regardless of TT, will probably start asking for at least 100 hours of multi. Where are you going to get those 100 hours of multi? Part 135? Well they will not hire you into their twins without 1200 hours at least or a couple hundred multi. Instructing? Probably, but the line of instructors to teach ONE multi student (gets you maybe 10-15 hours dual given) is several places long. We are expecting to have 1500 to 2000 hours dual given before we even get a multi student.
This mirrors what is going on at many other schools. Multi hours are so coveted that you have to wait a few years before you can give dual in a seminole and by then you already have your ATP mins.

Just giving the request CFI's perspective.