(in theory) 7 new job vacancies and 7 more free homes

No. Japan already has low unemployment and high overtime demands due to worker shortages, and property is already cheap (even in most cities other than Tokyo) due to a combination of falling population, complex construction/demolition codes, changing lifestyles, and a different view of asset classes/investment practices from the US.

It's kinda like how nobody likes inflation (other than certain kinds of debtors) but a deflationary economy isn't a good solution to that problem. This will have a similar effect because the people leaving are likely to be the most culturally assimilated/affiliated with decent language skills etc., but their departure won't alleviate the demand for additional labor in industry and agriculture so there will probably still be a net increase in short-term and illegal foreign residency.