Its inevitable that after 20+ years of serialization, the manga loses sales over time. That's just how the product life cycle works. One Piece is a manga aimed mainly at adolescent boys. As time passes, older fans will drop the series as they will find it harder to connect to, since its not their age demographic.
However, due to one piece's enormous length, new fans will be intimidated to even give it a try. Many people are hesitant to pick up one piece for this exact reason. I don't think the quality has really dropped(except for WCI, which imo was just a bad arc), One piece is just on the decline of its product cycle.
Which is why I think its more likely than not that OP will end in the next 5 years. There's only so much Oda can crank out of this. I think things will be becoming much more fast paced after this with arcs having less buildup and more fights.