So now it becomes way more expensive for small studios to come out with games that have online features. This is a huge win for big studios who will suck up all that market share.
Handing over a standalone server to the public is a massive engineering, financial, and legal headache. Modern multiplayer games rarely run on a single isolated program. They rely on a huge network of interconnected cloud microservices.
A single match might require separate proprietary systems for matchmaking, player inventories, anti cheat, metrics tracking, and database management. Many of those come with licenses that don't allow you to just give away the code for free.
Disentangling the actual game logic from these third party platforms like AWS or Epic Online Services requires months of rewriting code. At that point you're basically re-inventing the wheel on so many technologies that your costs go up exponentially.
Games are rarely built entirely from scratch by a single company and are usually packed with licensed third party software like proprietary network code, commercial physics engines, or specific anti cheat software. Because the studio doesn't own the rights to distribute these proprietary tools to the public for free then releasing a standalone server forces them to spend extensive legal and development hours stripping out the restricted code and replacing it with open source alternatives.
Releasing server code also exposes the inner workings of the company's technology. If a studio uses the same proprietary engine or backend framework for their active money making games then releasing the server code for a dead game essentially hands hackers and competitors a roadmap to exploit their current profitable titles.