It’s too good — for anyone. A noticeable chunk of the world’s most valuable retail space, along the streets of Manhattan, is sitting empty these days. In recent years, thousands of individual businesses have moved out, often replaced by national chains. Then even they move out, leaving “for lease” signs behind. The trend, reflected nationwide in closings eclipsing those of the 2008 financial crisis, is blamed on online retailers like Amazon. But observers say it’s also sky-high rents sought by institutional investors, content to write off vacancies as long as their portfolios’ value grows.