TOKYO—Nikesh Arora abruptly departed SoftBank Group Corp., two years after he was wooed from Google Inc. and had emerged as the heir apparent at the Japanese internet and telecommunications giant.

SoftBank CEO Masayoshi Son—who handpicked the Silicon Valley deal maker to be his successor—tied the departure to his decision to extend his stay at the helm of the company he founded for another five or 10 years, rather than handing over the reins.

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