Even if you dedicate yourself to this one cloud provider offering the technology stack you need, it probably won't solve enough of the challenges. I do think it's a sane investment to "move up the stack" towards paas services for most companies, for many reasons.
But it doesn't, in itself, provide the solution to all challenges. I'd even argue it would add challenges but on another level.
Then the second part about IDP's : There's need for findable documentation and to have a certain level of abstraction for developers. Also, the cloud provider will likely continuously change/improve their services, which can add to the chaos.
Then comes the other components like CI/CD, observability etc,etc. Most likely either from another vendor and/or have a different look & feel.
Having a single consistent interface where you bring all of your tech stack, tools, observability together without actually replacing all these tools is, in my opinion, a great investment.
Building your own portal is in general expensive, labour intensive, and difficult to maintain. So leveraging this as SaaS or self-hosted solutions could take away many concerns.
One of these developer portal solutions is Backstage. It's an Open Source platform where you could build your developer portal on top of, using plugins. You can check it out here:
https://backstage.io (disclaimer: I do DevRel for backstage)