@Mqrius So firstly, I don't really think they will. I wouldn't bet on this market if it was even near to 50% let alone above it.
But.
I think that second stage reusability is the best and most obvious way for SpaceX's competitors to close the gap to Falcon 9, especially start-up companies. Seems like if that happens SpaceX would probably respond.
SpaceX might be hoping that Starship's full reusability will itself make Falcon 9 and its competitors obsolete, but various ways that that doesn't happen can be imagined.
@JoshuaWilkes Fair enough. For me I don't see a 20% chance of that happening in any situation, especially considering they initially planned to reuse the second stage, but then dropped that in favor of developing Starship. I doubt they'd go back on that.