The pic isn't clear- but it almost looks as if this is a solid insert rather than a tube. Either way... if properly welded and heat treated a break here shouldn't occur unless it's a design flaw, or some outside factor- like a little help from gouging caused by chain derailment, etc. Then of course we don't know if the alloy is the best choice to resist breakage.

I'd be inclined to go for a new swingarm so I had some recourse in the event it happens again. Then wrap it with some sort of protecter to resist damage from thrown chains.