Not necessarily true. Back in 2006 during the transition from PS2 to PS3, lifetime sales of PS2 (100+ million units sold at that point) were comparable to PS4.
Yet Sony didn't hesitate to 'ignore' the massively successful PS2 & leave the past generation behind (Though I know God of War 2 released in 2007, as the final big first-party game). Once PS3 release in 2006, all their games are PS3 exclusive moving forward, with no PS2 version in development. Same thing happen during the transition from PS3 to PS4 in 2013.
Here's some examples of exclusive games in Year 1 by Sony's first-party studios, which weren't release on previous generation hardware.
PS3 (2006 - 2007) - Resistance: Fall of Man, Motorstorm, Genji: Days of the Blade, Untold Legends: Dark Kingdom, Lair, Heavenly Sword, Folklore, Ratchet & Clank Future: Tools of Destruction, The Eye of Judgment, Uncharted: Drake's Fortune.
No PS2 version.
PS4 (2013 - 2014) - Killzone: Shadow Fall, Infamous: Second Son, Driveclub, Resogun, Knack (The Order: 1886 were initially slated to release end of 2014, but got delayed to early 2015)
No PS3 version.