Actually this point was discussed on several occasions on the programs I have been listening to. Im trying to keep this less about my views and more of an "overview".
I am responding here as a political observer not as a "party member". I am a "The Economist"-style fiscal conservative, social liberal that doesn't fit either party. So, no vested interest in being for or against any party. Just my independent observations and analysis.
How many presidents who have lost an election have come back to win another? Most commentators thought his return was extremely unlikely.
None that I know of but two things: 1. Most Presidents at least in modern history who lost after one-term haven't wished to run again and 2. None of the past or norms apply to Trump. So, it is entirely up to him if he wants to run and he can win.
If he doesn't come back, it will be only because of health reasons or any legal issues he may face from his pre-President activities. Not because of his viability as a candidate. In fact, there is no Republican Party without Trump any more. The biggest losers in this election were the so-called never-Trumpers (Republicans who campaigned against him). Almost every one of his loyalists running for re-election won often at great odds (from polls). So, he delivered it for them. He has demanded and they have shown die-hard loyalty to him and they have not suffered in the elections for it. So, there is no viable challenger within the party against him let alone one that is trusted to deliver what Trump can for the party.
In addition, Democrats lost ground in both House (although they still have majority) and in local State elections. So Trump is seen as the only leader available to the Republicans capable of winning despite the changing demographics that goes against their traditional support base. His core base will believe he was cheated out of this election regardless of the truth. or logic.
Also does it not depend to an extent on how successful the next President is considered over the next term?
Depends on what success is. If the Senate majority stays with the Republicans ( as it is expected to barring two huge upsets in the Georgia run-offs in early January), they won't let a Democratic President do anything. They know how to fight (dirty if necessary) and win. Democrats only know how to hold hands and sing Kumbaya in a gun fight let alone bring even a knife. The Senate Democratic Party leader Chuck Schumer should be spending time playing Bocce ball somewhere in NY. He is so unfit to achieve anything that requires anything stronger than a strong tsk, tsk at the Republicans and throwing up hands saying there is only so much he can do.
In addition, Democratic Party is terrible at messaging and controlling the narrative that would let them achieve things against odds - both when they achieve something and when they are unable to. They are very much like total tech nerds that create a tech startup and think all they have to build is a great product and people will come and that Marketing and PR are bad and dishonest things. And when they try to do it, they are really bad at it. So, their success is mostly limited to whatever falls in their laps from their base afraid of the opponents than what they achieve.
One question, how stong do you view the competition, the other candidate, to be in this election?
If Biden wins, it will be because he was seen as not Trump (that the Democratic base was very afraid of) which is not a very complimentary thing. He, like most Democratic party members belongs to the old school of politics where you get into a room with the opposition, try to do some sausage-making behind closed doors with compromises and then go and have a beer later with them. He is going to get very little done especially with a hostile Senate.
The political maneuvering paradigm has totally changed (since Trump). Biden and the Democratic Party are totally unequipped to handle this. His only hope will be to have a growing economy through his term and let that translate into votes not much more. The Democratic Party is likely to disappoint most of his base in his term and will likely lose the House majority in 2022 which makes his rest of term a lame-duck. Trump will be undermining them from the outside every day controlling the narrative. I am positive there will be more press coverage of Trump during the next four years than Biden even if he is not in the White House.
The Democratic Party functions like a long running soap opera. Put people to sleep with a lot of activity but very little actually leading to concrete results relatively speaking and may be do one or two things of interest to their base and create a cliff-hanger and beg their base to bring them back so they can resolve it next season. These are like lambs to a slaughter for a Trump-like opponent, however well-intentioned their policies may be. And all their doings are easily reversed as soon as they lose power.