No; it's not.
Correct. Trump has other options.
The Commission on Presidential Debates is a very official sounding title for what is actually a self-selected board of leftist propagandists.
Trump should quickly announce the formation of The Commission for Fair Presidential Debates, schedule preemptive debates which precede each of the rigged debates, with settings, moderators and formats of Trump’s choosing.
Priebus should announce that the GOP is not satisfied that the CPD sponsored debates are impartial, and therefore has endorses the alternate CFPD sponsored debates, and will participate in the rigged CPD debates only if the Democrats participate in the fair CFPD debates.
If Clinton declines the invitations, simply use the empty chair routine and pass on the CPD rigged debates.
The only possible fly in the ointment would be if no network would agree to televise the fair debates. I really doubt they would all pass on the ratings bonanza, afterall, unlike the rigged debates, the fair debates would not be competing with NFL games.
However, as a last resort the Trump campaign could just buy the time - I can’t imagine money better spent.