Nintendo joining in and the games that they agreed to it really changed the nature of the conversation around this.
When it was just Microsoft, Sony could stand behind their "we have nothing to gain" from this stance and gamers would back them on it. Clearly they could see the benefits when it comes to allowing crossplay with some PC games, but they can dance around a PR line and avoid helping Microsoft before. Now that Nintendo gets involved, it becomes much harder to defend the decisions, especially when the advantages for gamers, developers, and platform holders in the long run seems better than the disadvantages.
Obviously it must have technical hurdles that may be tough to navigate to some degree, but Sony is getting greedy over online currency that just won't have the same value if there is a fundamental shift to how people are able to play online and that can happen without them if developers are able to unify every other platform.
I mean, an install base of 60 million is nice, but not every single one of those people play every game online. You combine all the players of a game across all other platforms and an online game can just prosper far more.
It's really an exciting prospect that I think Sony will fight hard against until it's absolutely clear that the tides have changed. I think they already have, but honestly, Sony doesn't necessarily lose out by changing their mind late in the game since it's basically like they just flip a switch and they get the same advantages. So in terms of hardware competition with MS or Nintendo, it won't hurt Sony unless they stick to their guns for years and the other systems make up lost ground, but I think if there are more games like Minecraft when Sony is the only platform with a different version of a game and maybe not getting all the current updates or even features immediately because of it, it will hurt the gamers and their version of the game. You get enough games like that and it will end up hurting the system.