Well, I guess it was either them or Oracle, given previous acquisition rumours.
Given Oracle's tendency to use pillows on processor architectures and bury them at night, Softbank is possibly the better of two evils? Jokes aside, I don't believe Oracle would evolve the A1s beyond what they need, and stagnate them for years instead.
Right now ARM is in a golden position to take server market pieces, higher compute densities with simpler i-set and good software support thanks to years of mobile and now Apple support (most language implementators wants things to run on Mac's so bugs and perf have been getting fixed)
If Oracle would've bought them, third parties (in this case including both Microsoft with Azure and Google) would probably have held back on ARM investments (not to mention smaller operators), Qualcomm being in lawsuit mode would also had sown doubt, maybe some other manufacturer would introduce chips in the meanwhile but they would be small and given time Risc-V might even pop up on the horizon as an alternative hurting ARM on server chances even more.
Now they're "independant", with better software support than ever, guaranteeing availability to smaller parties (at a time when many of those are outside the US and wanting alternatives) and a chance to benefit from vertical integration.