In general I think the opportunities in non-direct-to-consumer business will be great in the short term (there are a lot of inefficiencies that are hard to solve in-house by the big corps).
The problem is that it's much harder to sell to big companies than it is to sell to consumers. While a consumer may be receptive to an unreliable app with MVP-level functionality, a big company isn't going to risk the stability of its business operations on that quality of product. They're going to want stability, scalability and support, and they won't want to deal with a startup that's at high risk of not existing a year from now. One exception would be if the startup offered a unique product that couldn't be replicated internally. But if all they offer is the promise of better efficiency, what's the point of risking your business on it?
If a startup has survived long enough to achieve this kind of stability (like the already-public companies you cited), then it has a better chance selling to big corporations. But it's going to take a significant amount funding and growth for them to get to that point.
The companies that are seem to be winning the most at selling services to big companies are huge, established businesses like AWS, Microsoft and Google.