I don't know why it's so hard for people to reconcile the truth of the matter that the PC and Sony/Microsoft console ecosystems are not perfectly exchangeable in the same way that Xbox and PlayStation are. It's a viewpoint rooted in this utterly archaic framework of the video game industry, one that has long become totally redundant as the function and form of consoles and the experiences they offer begin to blur, yet the difference between consoles and other platforms remain vast. We've even see this within the console space: Nintendo specifically chose a direction that differed them from Microsoft and Sony's output precisely to reposition their market viability in a way that did not directly compete with and self cannibalise Sony/Microsoft's market. A lesson they learned with the GameCube and Nintendo 64.
Microsoft's business in the PC space is a little bit more nuanced as there's very specific benefits to them offering software to the PC market, but a lot of that just has to do with Microsoft's overall business approach in the medium (see: GamePass, and many GAAS titles). Sony doesn't benefit exactly the same way as Microsoft, but the can (and are) leverage the PC market at its core in the exactly like Microsoft. And that is by offering software to a marketplace that may not have purchased their hardware, without cannibalising their own market share.
Here's the thing; if Sony dropped all their games on an Xbox then yeah, the incentive to buy a PlayStation would reduce significantly. Because if you can get all your games on one box that offers a borderline identical end user experience...why not? Why bother with PlayStation if an Xbox is plug-and-play in largely the same ways. PC does not work like this though. As a gaming platform it is generally more expensive with larger upfront overheads. It isn't as accessible or immediate. It requires a more tailored, user intensive approach to benefit from. And, most importantly, the end user experience of using a PC and what it can/cannot do, and how, is so vastly different from any console that the two are not comparable.
Giving up your PlayStation for an Xbox (in the hypothetical of PlayStation games coming to Xbox) comes at a deficit of certain end user features and experiences (see: online friends lists, which itself is becoming more redundant as games become cross play), but giving up your PC for a console is completely different. The freedom and versatility and pure scope of function for the end user would be completely lost on a console. PC gamers jumping ship to PlayStation or Xbox are giving up a lot of exactly how they use their device.
And it goes the other way too. People who adore gaming on consoles do not want the higher upfront costs and lesser accessibility of a PC. An overwhelming majority of games already come to PC. There are more exclusives on PC (see: Indie marketplaces) than any console. PC is already littered with significant advantages over the console space (see: backwards compatibility, extensive library, peripheral support, etc). But people don't care, because people who love consoles want an entertainment box they can take hope, unpack, and plug immediately into a TV and it does everything they need with minimal user overhead. You buy a game, you put a disk in, and you're good to go. You don't have to fuck around with layered user interfaces or operating systems. You can just go for it. These people are not buying a PC even if their games are going to that platform too.
I don't doubt Sony genuinely sees other factors here that benefit them. I do think there is some value to someone playing Horizon on PC, loving it, and running out to buy a PlayStation 5. I'm sure Sony do measure some value in the minority of players doing this and continuing to buy the limited amount of exclusives on the hardware. I also think Sony are observing how the PC market responds to ports and, if the porting is to increase and the time between PS5->PC lessened, how they can benefit most with a launch (eg: opening their own storefront, offering subscription services, etc). They're late to the party but I'm sure they're still measuring numbers right now and trying to work out a long term approach.
But the absolute guts of it are as simple as this: people will not stop buying PlayStations to play PlayStation games even said games are ported to other platforms, on the condition that those platforms are inherently unappealing to them by virtue of their function and form. PlayStation gamers are, by and large, in the mass majority, not ever going to drop their consoles for a PC. Some PC gamers will buy a PlayStation specifically and exclusively just for the few games that they cannot get on their own platform, but the number of these people is surely significantly less than the PC gamers who do want to play these games but won't buy a PlayStation for them.
By releasing games to PC, especially late, Sony is simply capitalising on a market that will want to play their games but previously had no means to do so (willingly or otherwise). There will be zero cannibalisation of their own market or sales, as an overwhelming, vast, borderline absolute percentage of people buying these ports on PC were never going to buy a console at this point. People who wanted to play Horizon and Days Gone have done so. And when God of War '18 is ported it'll be the same story. Sony know, based on how the market has evolved and how Microsoft has now exploited it, that they lose literally, fundamentally nothing porting these games to PC. They just gain a wider audience and more sales. It's win/win for every person involved.
And that's why big name first party hardware publishers are incentivised to bring their software to PC.
