The business argument isn't there. Drupal for example has over a decade's lead and a developer community and pace of innovation that no private company of any size can match, let alone a startup. They are now competing with the last man standing in enterprise, Adobe. Even IBM are MS aren't really in the space any more. For business lower end, you cannot compete with Wordpress for price and developer availability. For personal, you would have a very tough job to even catch up with the mindshare of Wix and Squarespace, let alone exceed the products sufficiently to take their market, which is super low value unless you can bag most of it. You mention security a few times - do you think any privately funded CMS has a dedicated security team covering both core and community code as Drupal does? I will bet you a bridge that a private company's development resources get diverted to adding features in favour of security to sell the next upgrade.
As for IA/schema, one of the major reasons to use a dynamic CMS is that you configure your own because there is no such thing as a generic 'good IA' for any non-trivial content (recipes are probably the closest but even there most schemas are too limited for professional use). If you are working off a fixed IA, you may as well just use a low end SaaS. Drupal is particularly strong in IA, offering nested object modelling, workflow authoring, content versioning (now including staged sets of content so you can version sections or hub landing pages).
You then need to start adding considerations like BigPipe, or offline first access with eventual consistency for edits, and how all these play with access levels for granular pieces of content which may sit within pages depending on who's viewing, ditto contextual content. These are just a few things off the top of my head which CMS's start needing to provide.
If I was creating something in this space, I might look at small serverless modular apps performing simple use cases in a very polished way which could be used alone, or very easily integrated with major CMS's (including building, maintaining, and promoting those integrations). But possibly better to look at SaaS layers which sit on top and address digital experience and orchestrate content from multiple backends across multiple front end channels e.g. gamification, recommendations etc. A lot of money there and few open source solutions. (Even something as simple as buffer which just publishes from its backend to major social media channels has very little competition and is quite expensive. I'd imagine something which could simply manage a flow of content to and from social media, sites, apps could do well, even before you started adding in any intelligence around that content and your users.)