No one ever questioned Oda on his earlier 'off the cuff' estimates either so there was never any need for him to say 'I mean what I said.' I'm sure if someone asked after Alabasta, 'are you really sure we're halfway done?', Oda would reiterate the statement he had just made. Just breaking down the 'sagas' of One Piece:
EAST BLUE SAGA
Romance Dawn: Chapters 1 - 7
Buggy the Clown: Chapters 8 - 21
Black Cat Pirates: Chapters 22 - 41
Baratie: Chapters 42 - 68
Arlong: Chapters 69 - 95
Logue Town: Chapters 96 - 100
6 Story Arcs
100 Chapters
BAROQUE WORKS SAGA
Laboon: Chapters 101 - 105
Whiskey Peak: Chapters 106 - 114
Little Garden: Chapters 115 - 129
Drum Island: Chapters 130 - 154
Alabasta: Chapters 155 - 217
5 Story Arcs
117 Chapters
SKYPIEA SAGA
Jaya: Chapters 218 - 236
Skypiea: Chapters 237 - 302
2 Story Arcs
85 Chapters
CP9 SAGA
Davy Back Fight: Chapters 303 - 321
Water Seven: Chapters 322 - 374
Enies Lobby: Chapters 375 - 430
Post-Enies Lobby: Chapters 431 - 441
4 Story Arcs (or 3 depending on how you view the post-EL Chapters)
139 Chapters
PARAMOUNT WAR SAGA
Thriller Bark: Chapters 442 - 489
Sabaody: Chapters 490 - 513
Amazon Lily: Chapters 514 - 524
Impel Down: Chapters 525 - 549
Marineford: Chapters 550 - 580
Post-War: Chapters 581 - 597
6 Story Arcs
158 Chapters
FOUR EMPERORS SAGA
Reunion: Chapters 598 - 602
Fishman Island: Chapters 603 - 653
Punk Hazard: Chapters 654 - 699
Dressrosa: Chapters 700 - 801
Zou: Chapters 802 - 824
Whole Cake Island: Chapters 825 - 902
Reverie: Chapters 903 - 908
Wano: Chapters 909 - 1040 (Estimate)
8 Story Arcs
443 Chapters (Estimate)
Now, I know it's tough to break down 'sagas'; Everything in the New World has really been building to Wano. Each Island along the way has pushed the Straw Hats further toward a conflict with Big Mom and Kaido. There are some pretty direct analogues between the structure of the 'Kaido / Big Mom' saga and the Baroque Works saga. The individual arcs have gotten much longer - to the point that Wano, Dressrosa, and Whole Cake Island are the three longest in the series. Even 'short' arcs in the New World like Fishman Island and Punk Hazard are only 10 to 15 chapters shorter than mega arcs pre-TS like Alabasta and Skypiea.
There is an upward trend in saga length. Skypiea is a bit of an outlier, but I do believe it sets up things that will pay off toward the end of the series, particularly in Elbaf. I placed Thriller Bark into the 'Paramount War' saga because it seems to lay the groundwork. Kuma, Moria, etc. build into the War.
I think the final saga will be closer to the Four Emperor saga in length than any of the pre-time skip sagas. The Paramount War saga accomplished a lot in 158 chapters, but it asked more questions than it answered. In fact, it really laid the ground work for a lot of things that have been slowly been completed post-TS. However, I think the Four Emperors saga has actually opened up even more plot threads that will need to be closed.
With the sheer number of characters, the fact that we're not just one island away from Laugh Tale (We at least need the last Road Poneglyph, Lodestar, and Elbaf), and the fact that actually wrapping up storylines is the hardest and often the most lengthy part of the story, I think One Piece has further to go than Oda projected.
I'd also point to the fact that, as Robby mentioned, Oda often projects finishing individual story arcs within a certain period of time and often extends beyond that deadline. I'd point to the fact that in an arc like Dressrosa, the groundwork was laid for the battle with Doflamingo 45 chapters in; The Birdcage is deployed around chapter 745, but the action continues for another 50 chapters with several cool down chapters to follow. Wano took nearly 70 chapters to get to the action, with far more characters and still very little development for the main antagonist Kaido (We're clearly getting another flashback). The Tea Party on Whole Cake Island happens around chapter 860, but then the chase lasts 30+ chapters. My point is, the battles on Wano are likely to drag out until the end of next year. If you take this and map it onto the series as a whole, everything until Laugh Tale is really build up for the final conflict. Oda may project that he can wrap it up quickly, but when it comes down to actually writing these stories he's been setting up for decades, it's going to take him longer than he projects because he is a talented, dedicated author who wants to see his story told well.
I could see a final saga that matches or almost matches the 'Four Emperors' saga in terms of scale and complexity.