They also could have made quick and dirty port for PS4, they basically lost 1 year of PS4 sales this way (many people buying PS4 sold their PS3s and 360s). This is very similar to Gran Turismo 6 situation (also released in 2013) - but this game is still stuck in PS3 hell.
PS6 port will be based on PC version, or they won't bother with it right now and release it few years later. RDR2 was released 2 years before PS5 and we still have no info about native version, game just work under BC (with no upgrades).