I agree that for most adult learners, aiming for accent-free is likely to disappoint. However, my personal figure of merit is getting to the point where people are happy conversing with you in their L1/your L2, rather than wishing to swap to their L2/your L1, and any reasonably motivated adult ought to find that attainable.
no effort: your L1, their L2
easy mode: your L1, their L1
aim for: your L2, their L1
hard mode: your L2, their L2
(indeed, in my experience —past an acceptable baseline— people judge L2 speech far more by register and rhetoric than by phoneme production)
> children are very motivated
That, and much less afraid that they "might make a mistake"