If Trump appears certain to win in 2020, the Dems will likely chose who they want for 2024 as their 2020 VP pick.
Ask Geraldine Ferraro or John Edwards how that worked out. Anyone wonder why Tim Kaine hasn't stepped forward? Going down in flames as a failed VP candidate isn't the sort of thing aspiring presidential hopefuls want on their resume.
No, if the D choice in 2020 is to pick a "designated loser" they'll either go with a has-been like Biden or a lightweight upstart that they want to get rid of (Beto?). Either will keep the D options open for 2024.
The more interesting question is who does Trump run with for a second term? Hard to imagine Pence being the party standard-bearer in 2024. Steve Pieczenik has argued that Trump should dump Pence for Nikki Haley as his 2020 running mate both to take the wind out of a lot of D sails as well as to set up a viable heir apparent for 2024. It's an interesting argument and something I'm sure Trump will consider given that he wants to preserve his legacy after completing a second term.