I think one reason why Softbank would not be into this is that they would have to hold their money in stock for quite a long time. And since they took a freaking bath on their super fund including losing their shirts on WeWork, they are looking for cold hard cash about now.
I don't think ARM would be a great investment. Once the clients who have licensed ARM is pretty much mature (where we are starting to be at now), they aren't going to have a lot of large licensees handing over wads of cash. Sure, some small companies or new entrants will get into it.... but Apple, Qualcomm, AWS, etc already have their licenses.... as a potential investor, I'd not see a bunch of upside. Its probably also why Softbank is looking to cut them loose. Not sure if ARM gets a recurring payment based on # of chips produced by Apple in addition to the upfront licensing fee.