Re: Thrusters
They don't have spare boosters. They booked enough Atlas V boosters for the planned launches from ULA and ULA can't build more as the first stage uses rocket engines from Russia. And you can't exactly go to a nearby store to pick up a space russian rocket or two. Especially with Ukraine war and other issues.....
So once the currently available boosters run out, thats it.
In theory starliner can launch from Falcon9 as well I think (may need some modifications), but that will probably mean another demo or 2 on falcon 9s to certify that combination before it's man rated.
And since this is a fixed price contract, whatever extra flights are needed, Boeing is going to eat the cost. Last I heard starliner program is already about a billion in the red and Boeing is probably hoping to get the rest of the launches contracted for to go without issues so that they don't end up going another billion or two in the red.
They can always give up and not get the next milestone payments and finish off with just a billion in the red and probably alot of egg on their face. Not a good look if you want to get into the next government contract.
Or they can soldier on, do everything they have to do, no matter the cost, to make sure they manage to complete the contract, and hope they get another cost plus contract in the future so that they can drag that out to make back whatever they lost on starliner. This assumes that all the astronauts on this and future starliner flights are safe and nobody dies.