The last time initial jobless claims were this low, Ringo Starr ruled the pop charts

Goldman Sachs—the central bank could become the No. 1 shareholder in about 40 of the companies that make up the Nikkei 225 by the end of 2017, according to Bloomberg calculations that assume other major stakeholders keep their positions unchanged.

The BOJ’S ETF buying has come under criticism from opposition lawmakers for meddling in the markets, but Governor Haruhiko Kuroda has defended the program, saying recently that the purchases aren’t big relative to the size of the stock market. As of March the BOJ’S holdings amounted to about 1.6 percent of the capitalization of all companies listed in Japan. That compares with about 5 percent held by the nation’s Government Pension Investment Fund.

State intervention in stock markets has worked out well for some countries. The U.S. government spent $245 billion to prop up banks during the global financial crisis in 2008, earning a profit. At the height of the Asian financial crisis, in August 1998, Hong Kong bought HK$118 billion ($15.2 billion) of local shares to defend its currency peg, helping to fuel a rally that allowed it to dispose of the entire stake within five years.

Still, the longer the BOJ’S buying persists, the bigger the risk that market prices will detach from fundamentals. Assuming Goldman Sachs’s prediction for more stimulus is correct, the central bank could end up owning a quarter of Mitsumi Electric, a supplier to Apple, and 21 percent of Fast Retailing by the end of 2017.

Bulls have cheered the BOJ’S efforts to lift share prices, but the central bank is bound to reverse its intervention at some point, a potential source of instability that Sumitomo Mitsui Trust Bank says is increasingly on the minds of long-term investors. Says Ayako Sera, a Tokyobased market strategist at Sumitomo, “The biggest question will be: What happens when the BOJ exits?”

The bottom line The Japanese central bank has turned into a “whale” on the Tokyo Stock Exchange, but no one knows for how long.