Labour will have to have someone between Starmer and Burnham, assuming the latter is still the plan. They are going to have a nightmare in May and the stats around it will be used to see who is worth cohering around. It's safe to say that Your Party demonstrated a need but they've totally fucked it. Starmer will be gone by next Autumn; he'll cling on because his arrogance is the main thing he has in common with Johnson.
Reform's main issue is that they are a one-man party. It's the Nigel Farage show, they were a blip before he got involved. If anything were to happen to him, they're finished. But nothing will because life's not like that.