Well, I believe it is already answered here. I would try to give some answers of my own.
Sony is a business, and one that has struggled a lot in the 2000s. Sony cut 8,000 jobs in 2008, another 10,000 in 2012, and another 5,000 in 2014. In 2013, Sony sold its U.S. headquarters for $1.1 billion. These were all efforts to keep the company afloat. I believe it is in much better shape now, but it has gone through a lot. One of Sony's main divisions is gaming, and it cannot just afford to fail at that.
The Playstation 5, as well as most consoles these days, are probably sold at a loss when they launch. It has to make money somehow. Sony has a price strategy for the Playstation 5. Making the version with the blu-ray drive $100 more expensive may have to do with product differentiation or with the fact that Sony wants to create incentives towards digital sales.
Sony will not want to allow third party drives to be connected to the Playstation 5, perhaps for more than one reason. Of course, it would lose control over what can be connected to its console and potentially allow piracy to thrive. And this could be a disaster, especially because Sony would rely on the sale of software after selling the Playstation 5 at a loss.
Sony would also not want to allow competitors to offer cheaper blu-ray drives. Although the Playstation 5 may be sold at a loss at launch, it will eventually become profitable in the future. Sony does not want to give away that future profit by letting other companies benefit from the investment it is making now.
And, finally, want to keep quality control of the blue-ray drives. Low-quality blu-ray drives might become popular as they could be sold cheaper. And they might not be able to run the games at an acceptable speed. This could ruin the experience of using the Playstation 5. Many users would blame Sony, of course, and not the cheap obscure blu-ray drive manufacturer, thereby creating bad word of mouth for the console.
I do not see why external blu-ray drives could be good for users as well. They could ruin Sony's profit margins, which, in turn, could make it want to raise prices of games or accessories to compensate. If piracy is high, it could affect the decision of developers in making more games for the platform. A low-quality blu-ray drive may ruin the experience in playing games. A low-quality blu-ray drive that fails would have to be replaced, thereby increasing (and not decreasing) the total acquisition price of a Playstation 5.
So, I see a lot of reasons why Sony would not allow external blu-ray drives. However, I fail to see one single compelling reason why Sony should allow them.
