Given the timeline in this future scenario, it would bring Rebecca to an old age where it's definitely possible that Tess and Randall are going to visit her. Reddit user infinite_prism has theorized that "Maybe Rebecca has Alzheimer's and has days where she forgets that she adopted Randall and is hostile towards his whole family. Sad to think about, but that might explain why Randall is reluctant to see her."
There is also the possibility that Rebecca has already passed away and they are not ready to visit her grave for the first time. Regardless, we're not mentally prepared to handle another tragic This Is Us death.