There are two major dimensions to the operational cost weaknesses of open source. Neither is guaranteed to be meaningfully exploited by proprietary software in any particular case. Since we are talking about operational costs, this is almost entirely about data infrastructure software, not things like React.
First, there is a vast amount of supported operational tooling that is simply missing from open source ecosystems. PostgreSQL, which I still use extensively, is a perfect example of this. The code bases around this tooling are several times the size of the product they support, and it isn't the kind of code that most developers aspire to write despite its high value to the customer. Oracle and SQL Server have architectures that are as obsolete as PostgreSQL, they don't compete on the basis of being modern, efficient designs but on the basis of having dramatically better operational tooling. Cloud RDBMS attack a different aspect of this.
Second, is the operational infrastructure costs i.e. how much hardware is required to run a given workload. This is the larger threat to open source, particularly as the data intensity of business increases. PostgreSQL, Kafka, and Redis (I've used all three operationally) have several times the hardware requirements to deliver a workload in practice than is required with a state-of-the-art architecture. This isn't hypothetical, I've designed engines that replaced them when warranted. An 80% reduction in infrastructure cost is attractive when you are already spending tens or hundreds of millions of dollars per year on it. Some popular proprietary software is just as inefficient but that isn't guaranteed to remain the case (and I've replaced those systems too). Designing efficient software architecture isn't magic (see: ScyllaDB), you simply see very few examples of it in open source because it isn't a priority (see: use of managed languages for data infrastructure despite this being a known limitation).
The reality is that I can guarantee data intensive businesses who make competent use of open source that I can reduce their infrastructure costs by 4x with ease. You can bury a massive amount licensing and engineering costs in those savings.
Also, some companies are explicitly looking at this as a major "green" initiative. The wasteful infrastructure footprint to deliver a workload with open source is viewed as environmentally unfriendly, and this is pushing companies to consider proprietary solutions even if they are not sensitive to operational costs. That isn't a conversation open source is currently prepared to have but it is coming.