That's a fallacy. If we win the FA Cup (big if), to get to the final we'd have:
6 group games + 8 knockout games + 1 final = 15 games if the final is reached
In the FA Cup you've got:
5 knockout rounds + (potentially) 5 replays + 1 final = Minimum 6, maximum 11 games if the final is reached
In the League Cup you've got:
4 one-leg knockouts (3 if in Europe) with no replays + 1 two-leg semis + 1 final = Minimum 6 games, maximum 7 games if the final is reached
If you were to foresake both domestic cups, you're cutting out a potential 15 games (since you'd be in Europe and enter the League Cup later), as you'd have to play 1 in each to get knocked out, and replacing it with another 15 games.
I never see anyone saying "we don't have the squad necessary for domestic cups so shouldn't take them seriously" when the fact is that it's a like for like replacement if you go out in the first round of the FA Cup and League Cup.
Yes, there's the travel thing, and the placement of games, but by forsaking the domestic cups, you'd have rest time later in the season when others are competing.
It's entirely much of a muchness. I agree we don't have the squad for the FA Cup, League Cup and Europa League, but there's little reason why the Europa shouldn't be viewed as the main focal point in terms of cups, given that it carries Champions League qualification