Yes I fear that will be the case. The cupboard is bare and the Bruins are almost certainly going to have some tough seasons ahead. The high level talent that is sub-30 consists purely of McAvoy and Pastrnak, and even with Pasta I am not yet convinced that will be a long-term great player as opposed to 'just' a very good one. Swayman looks the goods too of course, but as with all young goalies the true test of his quality is yet to come. Vladar seems to be progressing fairly well too. Gryz are Carlo are decent, so to a lesser extent are Reilly, Lauzon and Clifton if they stick around. Coyle's best is good for a 3C, if he can recapture it. Lazar is a solid enough 4th liner. There is also Hall, but he's already 29, and I'd say it's no better than 50/50 that he even sticks around in Boston. Ritchie is out of contract and even if they were to re-sign him I don't think he'd do us much good. I think Kuraly and DeBrusk are gone, and Kuhlman is no more than a fringe player. Kase is all but done, poor guy.
In terms of prospects, 3rd line is the ceiling for Frederic, Studnicka has middle 6 talent but if he never develops the body and strength to play NHL long-term then he may never realize that ability at this level, and Zboril looks like a 5-6D. There are a couple like Steen, Hughes and Lauko who may well blossom into solid bottom 6 players. Vaak and perhaps one or two other defenders may become reliable enough depth defenders. With the more recent draftees we'll of course have to wait and see, but really only Beecher stands out as someone with any reasonable prospects of becoming a top 5 forward. The rest are far more speculative.
And that's about it - 3 top talents, 7 guys who are fairly established middling/depth players, a handful of kids who might become decent bottom 6/3-6 D players, and hopes of striking it lucky in a pretty lean pool of recent draftees. That's a depressingly small and under-strength group to work with.
The only Bruins who hold any significant trade value and who you would even consider parting with are Grzelcyk, Carlo and, and I will throw these in there despite them being controversial, Pastrnak and Marchand. However the last two are unrealistic, at least in the short term. So not much there either.
Bottom line is the Bruins are not in a strong position and they cannot trade or free agent their way back to the top the way the Penguins have tried and failed to do. That direction would lead only to further disaster. There is nothing for it but to go through at least a couple of lean years, secure as many high draft picks as possible, get those picks right, and hope to build a team of good young talent around McAvoy, Swayman, Pastrnak and at least a couple of quality free agency signings (inc. at least one top 6 center). As you say, I think it's going to take awhile, and honestly if the Bruins are back in a position to legitimately contend for another Cup before at least the 2025-26 season then I'll be surprised. There's a long road ahead, and that's why I'm convinced it's imperative they start that journey now and not in another 1-2 years time.