That's why the world runs on open source software. It's the cheapest way to share the responsibility of this common core of software. The only way really. Doing everything in house is stupidly expensive. Where open core companies fail is in attempting to monopolize their core of software. It's all the downsides of open (people copy your stuff) but without the upsides (you do all the work, at great expense). And by trying to mitigate the downsides, they actually make the problem worse.
The problem with open source companies is that most of their software rapidly loses its value after they start exploiting it. The type of open source company that insists on restrictive licensing (e.g. AGPL) and transfers of copyright tend to fail to create a software community around their stacks that would make them long term successful. That doesn't mean open source is bad for business, but just that that particular style of doing business isn't great. You throw out the baby with the bathwater. You incentivize your users to find (or worse build) alternative solutions. Basically, such companies being successful merely creates the financial incentive for others to do similar things. Launching such companies is easy with VC capital. Keeping them long term relevant and profitable isn't because inevitably a cheaper, more open competitor emerges.
That's why e.g. the Apache licensed ecosystem is highly successful. It regularly spawns new companies and they build on each other's success. And they end up sharing a lot of code. Which then becomes a shared commodity. Such is the nature of software. VCs don't like this because this doesn't produce a whole lot of ridiculously profitable unicorns. But it does create a lot of value. And there is plenty of money that can be made by developers working on such software. Which is why there is so much of it. OSS isn't charity mostly and there are a lot of well paid developers working on it.