I think at the very least, the P4 adds flexibility for people who have basic, single RCA-input class D amps but who have multiple sources, and want the convenience of a remote. I think there are more people using vinyl, CDs and a streamer and who don't want to change cabling around over and over.
After SO MANY products on the market with small, pretty questionable tube stages, having Fosi release the P4 and Schiit release the Saga 2 preamp within months of each other was a great thing. I wanted someone to also incorporate a phono preamp, or have as an optional card, but that doesn't seem to be in the cards.
Buckeye in the US has been developing an integrated amplifier for a long while, the work slowed by manufacturing issues and the demands of the rest of their product line. I think eventually Fosi and Aiyima may develop larger integrated, solid state amps, but they probably won't sell as many of those as their small and inexpensive devices.
Another plus of a separate preamp is that one can upgrade the amp or amps if power needs increase, e.g., you can use the P4 now with the Fosi V3 or Aiyima A07, and then jump up to a more powerful stereo amp or monoblocks if needed for a speaker upgrade.
My biggest gripe with the Fosi products is the need for their power bricks. While it does add flexibility, I still tend to prefer devices with internal power and just an AC cord. But even the Schiit Saga 2 was redesigned to use a large Wall-Wart.
