Corporates blocked out Basic Economy fares because they were initially non-changeable. I don't think it really had that much to do with lack of seat assignments, mileage earning, or upgrade ability. Some (most domestic?) airlines do now allow changes on BE fares for a fee but there hasn't been a movement to re-enable them in the corporate booking tools. Will be interesting to see what "features" they apply to basic business. I assume they will allow changes, possibly for a fee (though didn't they say no change fees ever!?), but lounge access and advance seat assignments are likely out. Lounge access will be interesting, of course they could just exclude the DeltaOne lounge, but that only impacts JFK for now. Will they also exclude SC access? That would only impact passengers who aren't Delta medallions or ST elites or Amex cardholders which probably isn't that many.
Removing advance seat assignments is probably the least offensive option, they could move to a BA-style scheme where top elites (DM/PM here maybe) get it regardless, lower-elites (GM/SM) get it at T-7d, and everyone else at T-24h, unless you want to pay. BA includes it on the highest 2 or 3 out of 5 fare classes but that's usually much more $$ than the single seat assignment fee (usually ~$150 on long-haul). There are clear benefits of this scheme for top elites and last-minute high-fare passengers as it prevents all the "best" seats from being preassigned by schmos on $2500 tickets purchased months in advance.