I like to think of this as a paradigm shift from businesses focusing on vertical integration where the same entity develops and maintains client- and server-side software and infrastructure and acts as the sole service provider, to a separation of service providers and software maintainers. It's a natural consequence of specialization and commoditization. We're already seeing this in cloud computing and databases. Case in point from OP: Dokku/Heroku. There is no reason why Dokku should have been a threat to Heroku. Ideally open-source tooling should only serve to strengthen someone like Heroku (whether they would have continued as a PaaS or pivoted to selling their software or other services around it). For a more recent example, I think the window for introducing another Netlify under the same model on the same scale is closing.
This is not bad news for business per se, it just requires a different strategy. If your service is solid, you should just be grateful that someone else can provide client software accommodating for users needs you may be neglecting or lacking the resources to accommodate. If you're a software builder it's a win to have a flourishing ecosystem of service providers for users to choose from, increasing the usefulness of your software and widening your market through the efforts of others.
If you have a proprietary API (openly documented or not) time is ticking.
Rent-seekers will struggle as their capabilities to hold customers hostage weaken. Businesses focusing on providing customer value (actual experienced value) should do well as long as they match their niche with their strengths.
Look at Tailscale. I think they get this.