One of the reasons Donald Trump defeated Hillary Clinton four years ago was by winning over most of the undecided voters in key swing states. That reservoir of potential support is much less exploitable this year—the numbers just aren't there. And, most polls show that about two-thirds of undecideds would lean toward Biden anyway—if forced to choose. So, if Trump can only pull from the remaining pool of about 2% of undecideds, it will be very hard for him to make up his current deficit of around 7 points.
Having said this, all indications are that this race is tightening—both nationally and in key battleground states. If the margin shrinks further over the next eight weeks (which it is likely to do), then the opportunity to "harvest" undecided voters could become more meaningful. Once again, as in 2016, this dynamic will be more important is swing states like Pennsylvania, Wisconsin, Michigan and Florida.
Nevertheless, the chance that undecided voters will ultimately 'decide' the 2020 winner is pretty remote.