I don't think this scenario is discussed, so here:
  1. We see a large hashrate drop (kinda like summer 2021)
  2. The drop is actually an entity starting to silently mine (mining blocks on its own without broadcasting blocks to others).
  3. They acquire more hashrate than the publicly known hashrate
  4. Their chain is always longer than the real network chain (if they keep up with the public network hashrate growth)
  5. After a while, they broadcast blocks, all nodes do a massive reorg because the protocol always favors longest chain.
The aftermath would be disasterous, since all transactions which occured during the time would be invalidated. Altough it's probably impossible to do, this would, in fact, harm bitcoin. This why, imo, it's important to have a steady hashrate growth, without massive drops.