Most likely that won't be the case. Even if sanction happen to OnePlus, it won't be affecting old device, the 1 that will be affected will be upcoming device after sanction.

Using Google and it services won't be an issue for those device pre sanction. What might happen is whether manufacturer will be keen to provide timely update to old device.

Yup if go for Samsung flagship & apple then surely won't face any problem. Reality is those willing to fork out the money to buy Samsung flagship or apple won't even consider flagship of OnePlus, honor and others china flagship. Exception will be those from vivo because of it excellent camera performance.

Just look at the number of pages of thread for flagship device. The popularity of the device speak for itself.

Their launch price is around the same or just slightly lower than Samsung. The thing is no people will buy those china phone at their launch price, mostly will just wait them to drop price massively post launch.

It's true that in term of support, Samsung is better. Camera wise video taking they are still the best and photo most probably only vivo is better. The rest of flagship like Xiaomi & honor are fail at their software level post processing although it come with better hardware and resulting in poor image/video quality.

Sanction wise I don't think it will affect google usability on pre sanction device like what I am saying previously. It's just a matter of whether they willing to provide support/update to those device pre sanction. China phone got a track record of slowing down update after new model launch anyway. If got sanctioned then it might be worst. Just I don't see the need for them to sanction other china phone brand. Those phone doesn't place a threat as market share is relatively small and their relation to china gov is not as deep as Huawei.