PC and/or a new player is their biggest threat. But they are not as big of threats as many people make them out to be. PC might bring Sony down to 85 million lifetime sales of a PS5 or PS6, but that's about it. Sure, there will be a slow trickle of kids growing up that make the PC their main platform, but the PC is slowly getting worse as time goes on. There are more and more storefronts these days. Windows 11 will likely be $10 to $20 a month to rent, with no way of outright buying a permanent license. As the west continues to decline western consumers will have less buying power than ever, (especially in the USA) making a costly PC a luxury that many can't afford. As India and China rise to the rank of 1st world countries, they will have consumers who will want to play AAA games, and the cheapest entry to that is a console.
Many people think that iPhones and streaming will do the same thing to consoles as they did to portables. But this idea has several flaws. First off the 3DS had a poor launch lineup, and was ridiculously overpriced at $250 for a tiny OG model. If NIntendo had just launched the 3DS with a 2DS XL model for $180 then the 3DS would have reached 90 million lifetime sales easily. The Vita was just a terrible handheld, with almost no exclusives, no backwards compatibility, and proprietary memory. It was Sony's Wii U, and sold poorly more due to being bad, than to having lost to iPhones/Tablets. The original DS was the PS2 of handhelds, and nobody should have expected 150 million lifetime sales to continue forever. Had Sony fully supported the Vita, and had Nintendo launched with a 2DS Xl, then the handheld market would be doing fine to this day.
But anyway, my point is that iPhones/Tablets are not as big a threat to consoles as people think, because they didn't exactly cause the downfall of the portable market. At least not by themselves.
The infrastructure for Streaming just isn't there, and probably won't get there for another decade.
IMO PC/iPhones/Streaming will not really threaten Sony. They will just take the marketshare that was previously held by the XB1. Gaming as a whole has grown since the start of this generation. Xbox sales have been nearly cut in half this gen. But you see Sony selling only 20-30% more PS4s than they did PS3s. So where did the rest of that growth go? Where did most of those Xbox consumers go? Well, they went to PC, and smart devices.
Edit: Consoles have grown more YoY than PC/Smartphones. 
