But the investment helps to ensure that it stays on schedule. The deal won't close til next Summer anyway, but there is a fairly immediate $3B that Sprint will get before approved.
Softbank is the one taking the risk here. Sprint doesn't really have anything to lose. Sprint could pay off some debt, rollout LTE that is better funded and maybe even buy out Clear and/or Cricket. If Softbank fails in 3-5 years, Sprint will likely just split off our be bought by someone with Sprint in much better shape.