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 he intends to step down from his role as chief executive officer anytime soon, his stated intention to retire in his 60s put the matter in focus when SoftBank sold six-year yen bonds this month.

While bond buyers have at times bemoaned Son's willingness to take on massive debt to fund acquisitions, they are growing increasingly concerned about how his eventual successor will manage the company's expanding investments.