> Around 60% of the money promised to the Vision Fund by investors other than SoftBank takes the form of debtlike securities that earn a 7% fixed return annually.
They get $70B and have to pay 7% fixed annually. S&P rate of return on average is 9.7%. Softbank could pocket 2.7% of $70B ($1.89B a year) by just investing in an index. With $70B you could stay solvent longer than the market can remain irrational, so you're operating with very little risk. (Ignoring all the difficulties in investing that much, etc)
Maybe SoftBank likes debt because even if they have no where to invest they'd still make $2B/year.
EDIT: This isn't a serious comment, literally just throwing numbers in the air for fun. I know very little about investing.