Why yen investors are worried about a future SoftBank without Masayoshi Son

Son, 60, said he wants to retire in his 60s but stay involved

Masayoshi Son is fond of talking about his 300-year vision for SoftBank Group Corp. but some Japanese bond investors are already pondering an earlier event -- what will the technology giant be like without its founder?

Son, 60, hasn’t named his successor at the company he founded in 1981 as a software distributor. While there’s no indication that Son has any intention of stepping down from his role as chief executive officer soon, his stated intention to retire in his 60s put the matter in focus when the SoftBank sold six-year yen bonds this month.

While bond buyers have at times bemoaned