It's not a bad idea in the abstract, but it's also not a trivial and cheap project. The author's example of Japan leaves out the little fact that although it's used on some new lines, and is being retrofitted on some old ones, the vast majority of Japanese rail/subway lines do not use platform barriers, and it will certainly be many decades, if ever, before a significant number do.
Besides the obvious construction costs, to make platform barriers work you need very consistent door positioning, which is maybe not so hard for new systems, but is a much bigger problem for old systems like the NYC subway (with multiple types of running stock, etc).
NYC has the additional problem that it has much higher (up to an order of magnitude in some cases) construction costs for public works projects than most other countries.
On the positive, platform doors are not an "all or nothing" proposition ... they can be added line-by-line, or station-by-station, as soon as necessary conditions are in place.
Certainly it's not a bad idea to study for the future, but it's also certainly not a simple project to undertake, and even if started soon, it would be a long time before enough any significant portion of the system is protected.