Well, in my experience, specialists for a given topic are significantly faster and deliver better quality than what generalists can deliver.

Sure, there's some advantages to being a generalist, e.g. you can tweak the API in the backend and adjust the frontend at the same time.
Or introduce a configuration value and integrate that into the deployment script.
Or even plan future features while keeping sane development practices in mind.

But aside from such cross-cutting concerns, I, as a backend dev with some forays into frontend and devops and planning, simply can't compete with anyone that mainly does frontend or ops or management. They'll throw something down in a day for which I'd need a week.

I assume, management people mainly favor fullstack-ish devs, because it gives them flexibility while planning. If you've got a frontend specialist and no frontend topics, you've got a problem. Whereas if you've got a fullstack dev, they can at least try to be useful with pretty much whatever topic you need solved.