The general interpretation of the issue states that allowable subsidies are, "those which are applied to research and pre-competitive development activities, and others which assist disadvantaged regions" [0]
Tens of billions of dollars in tax breaks and loan guarantees doesn't really seem like it could all fit under that definition, and indeed it looks likely that the WTO will rule that way as well.
The main problem with this case isn't just that AirBus received illegal subsidies. That can be true at the same time that Boeing also received them, making this a somewhat ridiculous thing to argue about, for Boeing to claim to be the aggrieved party here when any grievance or disadvantage has been offset by their own subsidies.
The end result will be additional tariffs imposed in both directions, making consumers pay more, at a time that tariffs and trade wars exacerbate a minor economic slowdown, increasing the chance that it will turn into something more.
[0] https://www.thefreelibrary.com/Trade+Liberalization+Under+th...