Watching Congress trying to pass a budget right now, you can see several blocs of Republican votes that they are trying to keep in line. Trump's direct intervention can usually get nearly all of them to fall in line, but there are still a few holdouts on most issues. No one else can get all of the factions of the party to hold together long enough to do anything legislative. If Trump falls, Vance has to first manage the turf wars in the Cabinet, and then he has to get the House Republicans all on the same page. After that the trick is getting low engagement voters who turned out for Trump to turn out without him on the ballot.
In Texas we just saw a Democrat win a state Senate seat in a district Trump carried by 17% to win by 14%. I need to check numbers, but several memes point to the Republicans having outspent the Democrats 10:1. The results point to low motivation on the Republican electorate, a shift in Hispanic voters, and high engagement by Democrats. Even with Trump in power the coalition that elected him is breaking up. Without him I can't see anyone holding it together for long.