Firstly, I believe the fears are founded; these fears are a good starting point, since learning and adopting a programming language is a big investment, and you should be careful when making big investments. To say they're unfounded suggests that they have no basis. Disagreed.

Secondly, I don't really feel like this sort of analysis does much to assuage fears, as Apple's business strategy is always going to take priority over what its engineers individually want. Apple of today doesn't have any obvious reason to just go and axe cross-platform Swift, but if that ever changes in the future, they could do it overnight, like it was never there. Could do it tomorrow. It's not much different than an employee getting laid off abruptly.

This is especially true because in truth Apple doesn't really have a strong incentive in the grand scheme of things to support Swift on non-Apple platforms. Even if they use it in this way today, it's certainly not core to their business, and it costs them to maintain, costs that they may eventually decide benefits their competitors more than it helps them.

There's no exact heuristic here, either. Go is entirely controlled by Google and does just fine, though it has the advantage of no conflict-of-interest regarding platforms. Nobody writing Go on Linux servers really has much reason to be concerned about its future; Partly because Google has quite a lot of Go running on Linux today, and given how long it took them to e.g. transition to Python 3 internally, I can just about guarantee you that if Go died it would probably not be abrupt. Even if it was, because of the massive amount of external stakeholders there are, it would quickly be picked up by some of the large orgs that have adopted it, like Uber or Digital Ocean. The risk analysis with Go is solid: Google has no particular conflict of interest here, as they don't control the platforms that Go is primarily used on; Google has business reasons to not abruptly discontinue it and especially not on Linux servers; there are multiple massive stakeholders with a lot of skin in the game who could pick up the pieces if they called it quits.

I believe Apple could also get to that point with Swift, but they might need a different route to get there, as Swift is still largely seen as "That Apple Thing" for now, by a lot of outsiders, and that's why I think they need to cede some control. Even if they did fund a Swift foundation, they could still remain substantially in control of the language; but at least having other stakeholders with skin in the game having a seat at the table would do a lot to assuage fears about Swift's future and decouple aspects of governance from Apple in ways that would probably ultimately benefit Swift for everyone.

P.S.: And I'm not singling Apple out here, because I think any rational company has to make tough decisions sometimes, but it's obvious from their past that they definitely don't fear changes of plan. Look all the way back to OpenDoc. Being willing to make bold changes of plan feels like it's a part of their company DNA.