My reading of the situation is that contractually McLaren are in a bind thanks to Mark Webber's tightly drawn driver agreement in Piastri's favour.
Mark who was once badly bitten by Vettel's easy avoidance of Multi21 agreement consequences, by clauses in his contract, seems determined this wont happen to Oscar with protection in a tightly worded agreement.
It's not No 1 status, but it prevents No 2 status.
Jacques Villeneuve has hinted at this today on Sky TV, he said:-
“It’s only the contract. They cannot stop Piastri going for a personal win but they can force him to help the team to win the constructors’.”
Villeneuve claimed that Piastri’s attitude will be: “‘I do not have to give up the win, and I won’t, because I am protected in the contract’.
“[Mark Webber, Piastri’s manager] did an amazing job. As a first-year driver to have a contract as a No1 - amazing! It has never happened in the past. Good for him.”
Certainly Oscar will play along to a certain extent, but should Lando leave a workable gap, as he did in Monza, expect the Australian to be going for it.
McLaren are unlikely to sack Oscar if they don't do the double, however we can expect Piastri to be off to another team in double quick time if they use undercut pitstops and other strategy tricks only in Lando's favour.
Unless Lando can really up his game and run at the front for the latter part of this season most team principals will regard Oscar as a future WDC, but unfortunately Lando as just a successful journeyman.