You know not everything Billie or fans say is intended to be taken literally right? It's just a positive sentiment about wanting the band to last, pretty sure almost everyone's aware it might not end up literally being true but most can see there isn't any need to say so.
Plenty of bands don't make statements about wanting to stay together as long as they possibly can, or even say they plan to stop when they get "too old", I think the fact Green Day do is certainly a good sign that there's a decent chance they'll last a long time even if (obviously) not a guarantee.