Full-stack developer knows some frontend, some backend, some sql. They are paid good money because they are convenient, not for their knowledge. A dev who does only one thing knows way more about it that a team of full-stack devs.

Full-stack devs earn a lot and will probably earn only more in the future.

This is mostly a result of tech advancements like cloud infrastructure, tools etc that takes all the hardest things from you - like managing a DB, implementing security, managing infrastructure, deployments etc.

You don't need deep experts because of it, generalists are perfect for quickly shipping new features.