One reason why P0, P1 and Photon are not recommended for new products is that the heart of them all - the WM-N-BM-09 "STM32F2x-WiFi-combi-module" - is manufactured by USI and Particle has no say in how long they will produce this device (i.e. recently there were shortages of them).
The nRF line on the other hand is a less specific item (i.e. standalone controller) and will most likely outlive the P0.
You can also read Will's reply to another thread that touches on that too
But there are currently no plans to abandon the Gen2 product line.