If HRR decreased then the midpoint would decrease.
Teams over the cap would have had to make moves to get under it. Trades, buyouts, minimal rosters, whatever it took. The cap for the next season is announced before the free agency period starts. Even if HRR and the cap declined, a scenario where no teams had cap room available for the next season is unrealistic. If some catastrophe caused that to happen I'm sure the NHL and PA would negotiate an exception to the cap rules to deal with it.