I have to agree that Sony stands the most to lose in such a situation. Though I do wonder if companies like Apple, Amazon, or Google will jump into the fray to protect their future ambitions in gaming. I'm very much surprised that Amazon hasn't made a successful bid at one of the larger remaining publishers. But perhaps a successful ABK deal pushes them to offer a lot more than they might have before?
I also wonder how this all might transform Sony in general. Clearly they've been looking to beef up their first-party already. Do they try to become more like Nintendo where their first-party sales are so strong that they can weather a storm of old third-party cash-cows suddenly going away? Is that pretty much the scenario you already laid out?
Most of my post spoke to the realities of what Sony will likely do - making moves to protect their SW revenue flow - in the event the deal closes as is. Sony's spent a lot of money since 2018 or 2019 in slowly but steadily moving their business plan into a direction that more closely resembles Nintendo; ideally their goal is to transition their current SW revenue stream from being majorly driven by 3rd party titles, to 1st party titles. Obviously, consolidating on their part gets them closer to that goal, but they also know they need to service their consumer with high-quality productions; the Nintendo business plan only works when your software output is very highly regarded. Sony's angle is to leverage both the value proposition of their titles, and expand them by beginning to increasingly publish in the PC space.
Keep in mind: Sony's need to increase their 1st party revenue is driven in large part to both offset what they feel will be a smaller amount of releases from 3rd party going forward (which we can plainly see is happening with Asian gaming publishers acting as the exception to this), so its less that they found the Nintendo approach necessarily more lucrative, rather that they needed to keep increasing SW revenue while 3rd party lowered the amount of full-price offerings they were seeking to pursue in the years to come.
Long-term, Sony was already set to transform, and they needed to: the business plan used during the PS3 and PS4 eras would no longer be sustainable. Third parties had drastically lowered the amount of titles they had published per year into the marketplace since 2010, with more and more titles coming out at F2P, and the overwhelming majority of MP titles have completely ditched the entry price and moved to F2P. The one factor Sony probably hadn't counted on is them finding success in their transmedia expansion of their IPs. Now that they are 2/2 on their major attempts (Uncharted at the BO, TLoU on HBO Max), not to mention the upcoming shows for Horizon with Netflix and God of War with Amazon, the ability for the Playstation division to grow their revenue streams beyond their current established lines is a very welcome one.
As for the external players who have a stake in the game's industry, like Amazon/Google, its hard to really say where they come down, but there are a few realities they have to contend with. For one, getting a new hardware unit adopted by consumers would be extraordinarily difficult. In 2012/2013, Amazon almost took a new console to market - I have played this console myself, I have developer colleagues who had nearly shipped launch software for it. Unfortunately, that was probably the last time anyone outside of the current console market paradigm would've been able to make a solid attempt at penetrating it. One of the factors that has made Sony's marketleader position so dominant (that honestly isn't even on regulators radar) is just how unbelievably difficult it is for users with PSN libraries to leave the ecosystem - MS has internal studies that back this up btw, and its one of the major reasons they even went in the direction they did with Game Pass. Sony's position becomes even more solidified as they continue to set records at selling software; if the current estimates hold that 65%+ of all SW sold in their eco is on PSN, then that means thats a record number of users who are buying things to keep them locked into Sony's eco. Very difficult cycle to break.
For the big 3, Amazon/Google/Apple, look for their next major initiatives to be in the gaming peripherals market. This is a massively lucrative sub-section of gaming whose profit margins are insane right out the gate, and whose userbase can flow to anywhere a user plays. One of the major reasons why Sony launched a new line of PC-focused gaming peripherals is because of how unbelievably lucrative this space has become in the last 3 years, how competitive this space is set to get in the short-term, and considering the adoption rate for things like controllers and phone-cradle controllers are taking off, expect to see more and more high-end gaming peripherals offered for anyone with a stake in gaming who doesn't wanna really engage in the console wars.
Apple's major push into gaming, like all their prior attempts, won't be as a dedicated initiative, but rather as an applicable vision for something whose use cases go beyond just gaming, aka their VR headset they intend to launch. Gaming will be a part of that drive, but like all of Apple's gaming initiatives, it'll be 1 part of the larger UX stories they intend for their device.