No we need to add a top-4 D regardless of whether Branny and Sandy "emerge".
Edit: Hell, for us to be a playoff team we need both that top 4 guy AND for one or two of those kids to take a step forward.
I think you're being overly pessimistic.
First 20 games: 4-15-1 48 GF 79 GA
Last 62: 29-2 7-6 179 GF 187 GA
We paced at a 37 pts pace and a shockingly bad -127 goal differential pace in the first 20 games. Those first 20 games, we had to contend with a slew of unexpected injuries in particular to two top 9 centers, terrible goaltending, and a playing through Covid outbreak that took a dozen players out of the lineup. That's not at all representative of what to expect going forward.
The last 62 games are likely a better indication, we still went through some tough situations, Batherson missed over half of those 62 games, Chabot missed 23, Norris missed 16, and Brown missed 14, just to state a few, so injuries were still about what should be expected, but we still managed to pace at a 85 pts pace, and a -11 goal differential pace per 82. We've added Giroux and DeBrincat, and only really lost C.Brown from that group. We also upgraded on Murray with Talbot, will be getting Pinto back, and will have Sanderson who if historical trends for top 10 drafted College D follow, will give us top 4 mins right away.
With a relatively healthy season, we should be a much improved team over the final 62, and that's not even factoring in the much better schedule this year (we easily had the toughest schedule in the league last year). Heck, even if Sanderson plays the entire year in the AHL which is doubtful, this team has a chance at playoffs based on the improvements it made over last years team, provided it can stay healthy.