There are more than 6 million bitcoin in OG wallets, that are waiting for Strategy and Blackrock to buy them over-the-counter. A supply shock makes sense if OTC trades' volume gets tightened, and they need to buy using a public orderbook. Until most of the OG wallets are emptied, there will probably not be a supply shock.
OG wallets can be lost wallets and even those OGs still have access to their wallets and bitcoins, it's still challenging for Strategy and BlackRock to get most of these coins. With Bitcoin ogs and whales, they can simply sell small part of their portfolios and it's enough in money to enjoy their lives. Likely most of Bitcoin OGs don't have need of selling all their bitcoins, and it's biggest challenge for Strategy, BlackRock and other Bitcoin Spot ETFs.
I agree that with time, the available bitcoins in OTC market will become less, as there will be less new bitcoins appears from mining, and Bitcoin OGs will have less bitcoins for selling in the future.