If one has the SERO plan, why would they give it up? If offered, why not accept it? SERO was never publicly advertised, thus the amount of people on those plans are limited. I see SERO has a loyalty perk at this point, since there isn't such a program at Sprint(or much of the industry) anymore.
SoftBank can shake-up the marketed plan pricing as they please(like their Double White plan in Japan), but if the new offering isn't much of a benefit to subscribers on older plans, why change?