Several separate questions to consider:

- The stability of the airline is not necessarily the same as the stability of the miles.

- Perhaps you haven't been watching foreign airlines as closely. Some of them (including "national" ones) have failed, then someone else took their place eventually (but they're really a separate airline, even if they carry the same airline code). Belgium's SN (once Savena, now SN Brusels) is a prime example I'm familiar with. Air Tahiti Nui is a more recent one that speculation teeters between going away and being forever propped up by their government.

- Earning ability through flying may be way smaller with foreign airlines. I don't know as much about Star, but in OneWorld, if you buy mostly economy, then AA gives you 100% of miles flown but BA gives you only 25%. AA gives you miles on virtually all fares sold on its website, but some of the foreign carriers give you NO miles at all for many sale fares!

- If upgrading is important to you, you can only upgrade if you're elite with the airline whose METAL you're flying. (Yeah, in theory there's an exception in Star, but so far all comments I hear about that is that it's more theory than reality.) If you're going to be flying mostly in the US, then that means you need to keep up elite status on a US airline, not a foreign one.

- How do you earn miles? Only through flying? If you earn a lot of miles through dining (iDine/Rewards Network), there are very few foreign airlines that participate in it (BA's the only one I've heard of). If you earn a lot of miles through hotel stays at chains like Candlewood Suites, though, which award a puny 1 mile per dollar spent to domestic airlines but a flat mileage (500 in the case of BA) per stay with many foreign airlines, that can be so better that it makes sense to have at least ONE such airline in your collection so you don't waste the miles from such hotel stays. (Well, there is ONE other option: Southwest gives a flat 0.5 credits at most such hotels. But that won't get you overseas, not even with ATA codesharing!)

- There's more than total loss of miles to worry about. There's also devaluation. And here's the grim reality of that: An airline which you'd tend to hate because it's just done a bunch of devaluations is most likely to wait quite a while before doing more devaluations, while the airline you love temporarily because it hasn't done devaluations yet may do them next quarter! Over my course of just a few years of earning miles and points, I've lost way more in effect through devaluation than I would have lost had ONE of the airlines you mentioned gone out of business and their miles gone with them.

If you're not concentrating in one airline for lifetime status or elite status, therefore, I would suggest that the best insurance against airlines going under is not to put all your eggs in one basket. (Certainly not all your miles earned by OTHER THAN flying.) Then even if an airline goes under and the miles vanish, you haven't lost it all, just a portion (just like with a big reward devaluation).