If Trump goes two terms, he will have completely transformed the party in his image. There will be a wave of Trump-ites in Congress who rode his coattails to power, one of which will probably be announced by Trump as his handpicked successor. That candidate will have the full weight of Fox News and the right wing media establishment behind him, and will be an absolute juggernaut.
I don't really see anyone in that list fitting that bill.
Also this. It's a much more likely outcome that the 2024 nominee (regardless of the 2020 result) is a loyal(-ish) Trumpist (whether it be Pence, Haley, or somebody else) who didn't run in 2016.
Not that anything regarding Trump makes a lick of logic or sense, but I would think that Fox News's love affair with Trump goes only as far as he's useful for cultivating their audience. If he crashes and burns, does Fox News remain the Trump Propaganda Network, or do they move on?
If Trump is somehow President in 2023, however (shudder), his official endorsement for successor will be hyped for weeks, and he'll make the announcement at a rally broadcast in prime time. I will personally barf if it comes to that.