Ok, I read your post as having agreed with them not having a singular flagship IP, but not with that being the reason for PlayStation AllStars' struggles. Regardless however, I still agree with the user you responded to regarding what they followed that up with.
PlayStation absolutely has benefitted largely from their association with popular 3rd party content, and that has been and still remains instrumental in their model for success. That's why they're kicking up so much fuss regarding MS' potential acquisition of ABK, whilst Nintendo's stance is basically "yea, whatever"... because associating PlayStation as the place to play titles like Call of Duty, or Grand Theft Auto, or Fortnite, or FIFA, or Street Fighter, or Final Fantasy... etc etc, was far more instrumental towards the mass market shift from Xbox 360 back to PlayStation 4 after a large migration in the other direction occured the prior generation as a result of Xbox managing to break the cycle of defacto 3rd party exclusives for PlayStation consoles. Today it remains why Xbox signing a timed exclusive like Rise of the Tomb Raider nukes the internet, whilst the equivalent types of deal on PlayStation consoles draws almost no scrutiny at all. There's simply a sense that notable 3rd IP belongs on PlayStation by default as a result of the associations built in earlier generations.
That isn't to say strong 1st party hasn't helped strengthen their position (along with Xbox imploding in the previous generation), but it's not at all coincidence that as PlayStation lost its grip on defacto 3rd party exclusives in the transition from PS2 to PS3, that Xbox immediately became a viable destination to switch to once PlayStation was no longer required for the next GTA, or Tomb Raider, or Tekken, or Resident Evil, or... you get the point I'd assume? Nintendo doesn't throw a ton of money at 3rd parties specifically to install a sense of "Nintendo Advantage", or "exclusively not on Xbox" clauses into deals they sign, because of the 120m or so owners of their hardware, the majority are buying it specifically for titles they produce. When PlayStation sells 150m consoles however, there's a huge overlap in the 30m or so players buying a combination of titles like The Last Of Us, God of War, Horizon Zero Dawn and the like. The prospect of buying a PlayStation instead for titles like GTA, FIFA and COD is extremely common, and remains key to their success to this day, which is why they fight so hard to maintain their perception hold on 3rd party IP. If a competitor ever put them in a 3rd party situation similar to that which they successfully managed to put their competitors in over the years, no amount of TLOU or God of War is preventing them suffering catastrophically in the market.