If he were a traditional candidate, the answer is that the party needs to have a candidate who not only can win, but also supports candidates for lower level elections. A traditional candidate would step aside and nominate someone to take their place. Not sure if the GOP convention rules allow it, but they could possibly even direct their delegates to go towards someone else.

Since Trump is a narcissist who doesn’t even get why downstream elections are important for building a political base of support, he’ll of course ignore this.