When you have one vendor they have no reason to give you any special attention. They are min-maxing you against all of their other customers and if you aren’t about to either Give them more money or take it away you fade into the background.
If you have two vendors, they have to pay attention. They have to and get to stay engaged.
If SpaceX is ahead or Boeing is ahead doesn’t matter. What matters if SpaceX is still at the table. If they drop to third or fourth place then they have a real problem.
NASA doesn’t want to give all their contracts to one vendor, and neither should we. That makes things easy this year but fattens then for the slaughter.
As long as Boeing is getting some noteworthy fraction of the contracts they stick around as a goad for SpaceX to try harder. And SpaceX does the same for Boeing.