False. I'm not saying Delta "would" lose in Japan. I'm saying they already have.
Delta is not now, nor has it ever been, and nor will it ever be, anywhere near as strong in Japan as the market's two dominant players, the hometown carriers JAL and ANA. NRT is no longer a viable "hub" for Delta - as evidenced by the fact that it has been steadily dismantled and its former traffic flows in large part shifted to SEA. Delta was always in a perpetually weaker third place (at best) position, and that was not sustainable. Many of us - not just me - identified this reality literally years ago.
Now, those unable to accept reality can continue trying to move the goalposts and constantly redefining what "winning" is, but it nonetheless still doesn't change the reality. AA and United have both also reduced capacity in Japan because they (1) were responding to broader economic and demographic trends in the market, and - critically - (2) had excellent local partners with whom to form JVs, giving them access to extensive (or at least far more extensive than anything Delta could ever sustain) beyond-NRT networks at effectively zero cost. At the same time, United leveraged its exceptional U.S. gateways to develop by far the most extensive and comprehensive network of nonstop flights overflying Japan to link U.S. megahubs with the principal cities of Northeast Asia. AA, in the span of just a few years, has similarly developed what is now an extremely impressive stable of nonstop connections from its U.S. gateways to those same cities - and has, by and large, closed the network gap with Delta in the nonstop U.S.-Asia market, and, by and large, closed the network gap with Delta in the beyond-NRT-Asia market through the JAL JV.
Once again, bottom line: like with the rest of the world, Delta today has little competitive advantage - on a network basis - in the U.S.-Asia market compared to AA, and essentially none at all compared to United. And building a line hangar to do overnight work on 757s going to GUM isn't going to change that.