Those are three very legitimate concerns, and there are others, too. I'm not trying to lessen the importance of these concerns, since I worry about these things, too.
1. I'm not big on thinking a player or team's most recent production level is who they are and will be, going forward. We have dropped off, late in the season a couple years in a row, so it does make you think about why that happens.
2. Losing O'Neill is a big one, but he did miss about 50 games and was just 5th on the team in PAs w 473. We are also losing more than 473 PAs from these guys: 278 Dom Smith (.706) 223 E Valdez (.633) 158 McGuire (.575) 96 Jansen (.623) 93 Dalbec (.410) and 75 Cooper (.455) This is about double the PAs lost by O'Neill. While there is no certainty Narvaez and other back-up catchers will outhit McGuire and Jansen's 2024 numbers, the bar is pretty damn low. Replacing Smith/Dalbec/Cooper with more PAs from Casas could easily make up for the loss of O'Neill, all by itself. (Casas has good splits vs LHPs, too.) The 2B offense was putrid, until DHam and Romy started playing more often, Replacing EValdez on O and D should be a big boost. If Grissom hits .465, again, he won't get the 114 PAs he got in 2024. Replacing O'Neill's PAs will likely come from more OF PAs from Rafaela, which is not very promising, offensively. We could see a 200 point drop, there. The only current other hopes we have are more OF PAs from Refsnyder vs LHPs or Anthony and Campbell.
3. I'm not sure about Devers and his shoulder. I hope the docs got it right with no surgery. We can't afford losing him, or having him play hurt, again. I guess Campbell could play 3B, but there is no replacing Devers' bat.
Another big question is SS, particularly Story's health and how well he may do on offense. It's been a long time since he was a force on O, and we could use a power bat from the right side. It's not likely Wong will keep hitting this well, although he is trying to put on weight.
We need to rely on the younger players (Duran, DHam, Abreu, Rafaela, Grissom and others) to do well or improve. A healthy Yoshida could help, too. We have about as many question marks as everyday players on the 40 and off the 40, but we do have a lot of depth and choices, if someone gets hurt or falters. (Guys like Devers and Casas are not easily replaced by the in system depth we have.)
Devers, Casas, Duran
Story, Abreu, Rafaela
Wong, Yoshida/Refsnyder and the 3-headed 2Bmen: DHam/Romy/Grissom
Campbell, Anthony and Mayer
That's 15 players of 9 slots, plus these 3 for catcher depth:
Narvaez, Sabol & Zavala