According to Richard Baris of the People's Pundit (best MAGA pollster in the business), 30% of MAGA voters say they will write in Trump's name if he's not the nominee. Another 5% say they won't vote at all.
The GOPe understands this. They know if they can deny Trump the nomination the Republican Party will forever become irrelevant and we'll be living in a single party system.
They don't care because we already live in a single party system!