The good news is that the PS5 and the XSX aren't too far away from each other in terms of hardware power and architecture. Further, developing on XSX also mostly works out of the box with DirectX, which means it is easy to also get the game running on PC. It's relatively easy to build a game out of a generic system and task our engine programmers with getting that generic system working on each of our target platforms.
The difficulty in multiplatform development comes from trying to get the same generic system to run on drastically different hardware power profiles or architectural differences (commonly known as the Nintendo problem). If we have a game that assumes the player's hardware have at least 16GB of RAM and an 8-core 3.5GHz CPU and we suddenly have to fit that game into 4GB of RAM and a 1GHz 4-core CPU, we've got to make a lot of drastic changes in order to get the game running at all. I'm fond of saying that porting PS/Xbox games to Nintendo hardware is trying to get an entire Honda Accord to fit inside a Mini-Cooper.
The general rule when estimating the cost of making a change is how early during the process the change is made. The earlier in the process the change is made, the cheaper the cost of the change. Making a change to a movie before it's cast and shot is much easier and cheaper than making a change after the filming is complete. Making the decision for a project to go multiplatform from the jump means that the entire project will be built with maintaining multiplatform stability as a major goal. This means that further decisions will be made with that goal in mind - the team might spend those resources elsewhere instead of optimizing for certain platform-specific hardware features.
As for why Square-Enix decided to go platform exclusive with FF16 and the FF7 Remakes, it is likely that Sony offered them a seemingly-better deal. Most third party publishers get a standard deal with the platform - the platform takes a 30% cut of all of the game's revenue, the game must pass certification, the platform gets some kind of exclusive content for that version of the game, and so on. If the platform wants to get an exclusive, they offer a better deal than that - maybe Sony agrees to pay for some of the marketing of the game, maybe Sony takes a smaller cut of the revenue, maybe Sony waives the certification costs on the publisher's next five Playstation games, and so on and so forth. These concessions and incentives are certainly worth considering. Sometimes they work out well for the third party, like Insomniac's Spider-Man games. Sometimes they don't.
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