There's some cost involved there--especially if you're offering hosted runners (you need some capital to buy infra to run jobs)
There's also advantage of limiting how many companies you share your code with. If you're using GitHub, you're already trusting them with your source code so GitHub Actions doesn't require more trust or an additional entity to manage/pay
If you rely on "Trusted Publishing" you are assisting Microsoft in making a moat for their CI platform.
Use cryptographic signatures, not implicit trust in a hosted platform.
This change is a very weird one. Because the teams that are capable of setting up their own runners have the means to easily jump ship to another CI platform and skip these fees altogether.
(DECLARATIVE CI WHY?????)