Ok, i'll elaborate a bit further.
2021 is when the driver market is going to flow again.
If the rules indeed are going to change in 2021, that means that teams themselves are looking in two ways for drivers:
1. keep one or get one with sufficient experience so they can develop the 'new' car(s)
2. if they don't already have one, attract a driver that is going to be their star driver and get them forward.
for the drivers themselves, this means potential opportunities to get into a team which has potential, or at the least, has more potential for success than the team they are currently finding themselves in.
Mercedes' dominance is naturally going to 'end' sooner or later and we already see that Ferrari and Red Bull are closing in.
There's some life in it, but then the competition is going to kick in. Not just that, but with the renewed design approach, teams need to follow different philosophies and as such changes the field and philosophy of the car's design. Lewis PROBABLY is going to grab his 6th title this season. Chances are still reasonably high that for 2020, he's still very much in the running for another title, so let's be fair and say it's believable he will approach his 7th title in 2020. It would make him equal with Schumacher in titles. That means if he wants to surpass Michael, which in the end he does, he needs to go on in F1, so i don't believe he's going to retire in 2021.
But he has had his success with Mercedes more than enough by now, even with Mclaren if we are honest, and i'm sure he's open for a new challenge. He has actually mentioned before Ferrari does interest him, and i'm certain Ferrari also has interest in Hamilton. If there's any moment where Hamilton might go to Ferrari, that would be, without doubt, 2021. And i'm pretty sure Vettel is not going to make the 2019 WDC, so he's going to give it one final run for 2020. Won't make it, i think, and Vettel will retire from F1.
What does this have to do with Ocon?
Well, it means that quite frankly, Mercedes' seats are occupied until the 2021 season - after all, Bottas is comfortable where he is, and even though i do believe that as of 2021, he's logically going to be open for a transfer, Mercedes' is going to need him too much, and since he has his career to thank to Toto Wolff for quite a lot, they'll make him an offer he won't refuse and as such, 1 seat is already occupied for 2021 and on.
It will be one Mercedes seat open in 2021 then.
Who will take that seat then?
As said, Vettel undoubtedly will retire, so no Vettel in there.
What's to choose from then?
Bottas is a strong driver, but not a superstar driver, and they need a superstar driver.
Who fits that criteria? Well, Verstappen does, Leclerc arguably does, and yes, Alonso does, but i seriously doubt he's going to make a comeback by 2021. If Ocon wants to have a chance on that seat, he NEEDS to be in a racing seat in 2020 somewhere and be competitive and show his potential. Right now, he simply can't. Where can he go in 2020? Well, again, Williams is occupied. The only Mercedes-powered team left is Racing Point, but even if Perez would leave, i doubt RP has interest in Ocon. That means Ocon needs to go to a non-Mercedes racing team.
The only team that for me looks like an option is Renault, to replace Hulkenberg. And i do believe, that if and when he does, that does have influence on whether he can go to Mercedes main team.
Thing is, there is no Mercedes - connected seat available next year, and Ocon CANNOT go another year without being in F1 if he wants to continue in that class, especially if he wants to have the slightest chance for a Mercedes seat. Wolff already stated they're not going to keep him from going to a 'competitor' team if he can't get a Mercedes connected seat - which again, there isn't. SO, IF Ocon is going to get a seat next year, it won't be with anything Mercedes, and probably will be with Renault, and if that happens, its an easy relationship termination with Ocon for Mercedes. So, end of Mercedes opportunity. Even if there would be a Merc seat at RP for example, it's still arguable whether he'd get to Mercedes by 2021. And again, that is IF Hamilton actually leaves.
And that's not even delving one more time into the fact that Verstappen is more experienced, matured and proven faster than Ocon, so again, they'd very much go with Max over Esteban. Again, it's a win-win for Mercedes if they get Verstappen because it also takes away a competitor from the enemy teams. It would render Red Bull more or less powerless if they'd manage to do that, depending on what Honda and Red Bull cook up in the coming season(s).
It would also take away the possibility that Verstappen goes to Ferrari alongside Hamilton, if such an opportunity arose, which i don't believe will happen as i expect Leclerc to stay with Ferrari for atleast to 2022 or 2023.
Renault is french, Ocon is french. Hulk at the end of his career, Ocon at the beginning.
Ricciardo is fast and experienced so Renault can afford to 'gamble' with Ocon.
Abiteboul is just like Ocon so that would make a 'perfect' friendship, and Renault would love having a frenchy at their french team. Ocon is also quite fast, so would be a great choice. After all, who else would they attract for 2020 except for 'gambling' with Kubica who they rejected before?
To me - Ocon in F1 - i think it's clear:
Ocon 2020 in Renault F1 Team.
It's the only possibility. If he doesn't go with Renault in 2020, he will drive for another racing class altogether in 2020.
Maybe remain a development test and reserve driver for Mercedes, but WEC/LM or Indy for sure.
Whilst being semi 'silly season' right now, if Perez calls it quits in 2020, which might or might not happen,
i think Russell will get promoted to Racing Point. Again, even if that seat becomes available, i don't think RP is open for Ocon to take that position. That means Williams has an option but Ocon would commit career suicide if he goes anywhere near Williams. Mercedes would want Russell at RP to see his development progress and mold him, guide him, and test him for one day replacing a mothership driver - that would probably be Bottas, and would be around 2022 or 2023. That means Russell will get 2 seasons at RP to prepare himself, which seems reasonable to me.
Why does this still import Ocon? Well,
By then - 2022 or 2023 - Ocon will have had all the time in the world to show his potential and make himself 'desired'.
And by then, after driving for Renault for a season or 2, he could find himself either staying at an improved and competitive Renault (being at the right place at the right time contrary to what is happening to him now), find an easy route towards Mclaren to replace Sainz (through Renault), OR get back to Mercedes and replace Bottas if he proves to be better than Russell. Or in a dream scenario, find himself replacing LeClerc by 2022 or 2023.
So again - Ocon in F1 must be with Renault in 2020. It's his only choice.