I'm not certain. In an aging society, the thing they'd need are young laborers. People with 30 years of savings are unlikely to be either young, nor laborers.

Maybe those wealthy immigrants are very talented people who are very good at, say, managing an insurance company or researching pharmaceuticals, but Japan has a glut of 55-year-olds who can do that stuff very well. They're running short of young people who can be nurses, junior machinists, and construction workers.

In a macroeconomic sense, wealthy people might be able to pay for the services they need from their own pockets, but functionally it's not that different than being unemployed (unless they choose to work). They would be almost purely adding demand to the economy. The wealth they are bringing represents supply that they added into the economy in the past and abroad, and now they have a wallet full of paper with which they can buy services for the next 30 years in Japan without having to do any further productive work.