The risk the debt mountain poses is the concern most often cited by investors, says Mr Lane.
Some of this debt will go to the separately listed mobile unit and may be "non-recourse",
meaning that SoftBank may no longer be liable for it if the subsidiary defaults.
For now, Japan's negative interest rates, adopted by the Bank of Japan in 2016 as part of its quantitative-easing programme,
allow firms to borrow without worrying too much; they should remain for at least a year.
Critics complain, too, that Mr Son's plans are wildly ambitious but unclear; and that "key man" risk around him remains too high.
That suggests hopes for a re-rating of the parent's shares may go unanswered.
Investors in the mobile arm might face an additional governance risk,
if the subsidiary tries to please its parent and majority stakeholder rather than ensure returns for minority shareholders.
As for the Vision Fund itself, Mr Son was criticised for failing to distance himself from Saudi Arabia
when its crown prince, Mohammed bin Salman, was linked to the killing of a Saudi journalist in its embassy in Turkey in October.
Startups might be more reluctant to accept Mr Son's cash as a result, some rival tech investors say.
Early divestments from the fund seem to have worked well, however.
When Walmart bought SoftBank's stake in Flipkart, an Indian e-commerce firm, in May,
it reportedly paid almost double what SoftBank did less than a year earlier. SoftBank invested in Uber at a valuation of around $48bn;
reports suggest its valuation will be north of $100bn when it lists on the stockmarket, possibly in the first quarter of 2019.
Next to that kind of money, the telecoms business, expected to make 700bn yen of operating profit this year, looks like small fry.
1.be liable for 有…责任；有…义务
He is liable for the fault of his son.
Chris is so ambitious, so determined to do it all.
3.minority shareholders 少数股东
The election of directors shall fully reflect the opinions of minority shareholders.
4.be reluctant to 不情愿
She's reluctant to take all the credit.