I suspect there will be another choice, as they will announce Biden's decision to retire (not contest the election) just before the Democrats conference in two-three weeks - which will confirm the (new) presidential candidate nomination.
This debate was held much earlier than usual before the November election. There is time for another debate in the usual timeframes, between the two candidates that will be contesting the election for each party.
Because of Biden and the Democrat's failure to succession plan, Trump is in pole position, with everything to lose...