Hard to answer.
As for their biggest competitor, I'd say: In Japan, definitely Nintendo. In Europe, I'd say PC. In North America, Xbox and Nintendo share that spot.
However, as for the biggest threat, I would say mobile gaming. And that's not just for Sony, but also for Xbox, PC and Nintendo.
Case of point: I gave my nephew my Switch and Super Mario Bros 3 to play. He didn't like it. I thought it was because of the outdated graphics, but he answered that it is too complicated. And yeah, he died 3 times against the same first Goomba, both in SMB3 and in SMB (which he did dislike because of the outdated graphics btw). I gave him the Dragon Quest Builders demo, but he couldn't figure out the controls and couldn't leave the first room as a result. Ironically maybe, he's very good at Snipperclips Plus instead. My wife, who never played Videogames outside of her Smartphone before, plays Super Mario Kart 8 Deluxe, but zigzags more than a sleepy drunkard. She's just unable to understand that the kart will drive forwards by itself without any input from the player (and no, she doesn't have a driver's license!)
They, and probably a whole Generation along with them, are more used to play things like Hungry Shark, Hay Day or some Match-3 clones. Things that don't need any effort or are just puzzles. Console or PC games need a minimum of effort an rarely are just puzzles, and thus seem to much already for them. Unless we can teach them how to play and what makes a good video game, I fear the market for consoles and PC gaming will shrink in the coming years.
My son started playing God of War when he had about 3,5 year and is about halfway done. Have finished Knack 2 4 times, finished Lego Marvel Super Heroes. Yes he need to put a lot of effort on the game, but he is learning and is quite good at playing console even if he also plays a lot of ios games.
duduspace11 "Well, since we are estimating costs, Pokemon Red/Blue did cost Nintendo about $50m to make back in 1996"
Mr Puggsly: "Hehe, I said good profit. You said big profit. Frankly, not losing money is what I meant by good. Don't get hung up on semantics"