Exactly and as Sprint increases their coverage, so goes their marketshare and ARPU. The only thing really holding back Sprint's growth is their coverage. SoftBank can fix that. SoftBank also has churn in line with Verizon and AT&T at around 1% so as Sprint's "problems" are fixed, they'll be able keep the customers they get.
The market in Japan is completely saturated with 127M
subscribers out of a population of about 127M. Growth between all of the carriers in Japan was only 0.6%. They're done.
This is a logical move for SoftBank who is having a lot of trouble stealing customers from NTT and au/KDDI. With improved Sprint coverage, swiping AT&T and Verizon customers wouldn't be nearly as difficult, especially if they keep their aggressive data plans or adapt the sliding model for data usage they have in Japan.