The offence was sound, last year. While we may not replace Gregorius' bat, others may step up. Yeah, I know, too many 'mays'! I'm not as concerned about the age of the offence - with the exception of McCutchen, the other confirmed bats are still in or nearing their peak years.
McCutchen and Kingery are the wild cards for me. Cutch is 34 this year. After an injury-affected start to the season, his offensive numbers improved steadily as 2020 progressed. If the first two weeks of the season are excluded, then from Aug 18 to Sep 27, he had an OPS of .829. I wouldn't bet on that being sustained next year nor him being injury-free. However, I am 'hopeful' that he won't experience a huge drop-off in offence.
I am not as 'hopeful' regarding Kingery. He's 27 in 2021 and, ordinarily, we could anticipate some improvement. A healthy Kingery, i.e., capable of repeating his 2019 production, may largely fill the offensive hole left by Gregorius. However, Jim Peyton over at PhuturePhillies has remarked a few times about the severity of Kingery's bout of COVID-19 and the longevity of the side-effects. Maybe I should just hope that he eventually makes a full recovery, rather than expect that he will contribute significantly in 2021. I hope I'm wrong, too.