The Japanese economy and especially its financial markets were completely insane in the '80s. As in, large scale structural problems; some investors were "too important" to lose money so the brokers would reimburse them. See also the "Lost Decade(s)". (https://en.wikipedia.org/wiki/Lost_Decade_(Japan).) The Nikkei's decline cannot be described as a normal "downturn".

But, useful inferences for the current market are left to the reader.