I suspect Nissan's decision to leave LMP1 is partially due to suddenly getting major success in LMP3, the super-cost-capped, mostly-renewable mini-prototype class that mostly races in ELMS and similar regional-level series. Oh, and it's managed to secure the driving services of Sir Chris Hoy, following his successful debut season with them. (If you've been to the cinema recently, you may have seen the advertising for this already). With some easier marketing and development wins there, Nissan have better parts of sportscar racing to invest than an LMP1 project that was going to take a lot of work in order to reach similar levels of success and marketability.
Don't worry about any other manufacturer doing the same trick. The FIA restricted LMP3 to four manufacturers, and the others are small enough that LMP3 is already their biggest series.