For the first ~15 years of my career, multiple teams, the model was always that engineering was responsible for all the test coverage of all areas. It was wonderful. A separate dedicated QA team will never, almost no matter how good they are, have the same deep insight into the code both big picture and small picture. So they won't be able to exercise it via tests as well as the engineers who wrote it. I always thought, this is the only way that makes sense.

To my surprise in subsequent roles, I've seen many development teams, even good ones, who don't have that deep instinct to break their code via tests. Which was very alien to me. I've seen enough cases now to realize it's somewhat common. For such teams, separate QA engineers are a must.

I still feel having quality responsibility in the engineering team ultimately can offer the best quality outcome, but only if you can build a team with the right attitude about testing. If not, having a separate QA team is vital.