Since the market has been growing every system that was less profitable than its predecessor in the same timeframe is to an extend failure.
The biggest fails where PS Vita, Wii U, GameCube, Xbox One and N64. Not saying those are failures since especially the OG Xbox and N64 had very impactful games, also Wii U provided Switch with a lot of ports that sold millions. But from a business point of view it was not what they had envisioned.
Even though the gap between PS4 and PS5 is likely to grow, it's still a success since it's much more profitable.
Please excuse my (probally) poor grammar