We shouldn't necessarily assume that the x86 patent war chests are legally sound. Rather, Intel and AMD have a mutually assured destruction cross-licensing arrangement and they need each other to stay viable to fend off antitrust regulators. But they would prefer not risking an unfavorable precedent by actually wielding their patents in court, so their deterrent operates more on the promise of protracted and expensive litigation, rather than on the promise that Intel would actually win against an upstart CPU vendor.