Elon Musk has repeatedly offered to allow other manufacturers to use the Tesla supercharger network, he's been saying it for years:

https://www.cnet.com/roadshow/news/tesla-elon-musk-open-supe...

https://www.gearbrain.com/tesla-supercharger-accessible-othe...

He's even fine with them using adapters for Tesla's plug, they just have to pay "their fair share" to use the Tesla superchargers. Of course the devil's in the details who knows what a licensing agreement like this would cost.

And how much it would cost when you're trapped

It’s not just the amount of capital. Tesla put its capital at risk when the risk was much higher. When it’s company was smaller relative to scale. Volkswaggen got a free ride to see if the experiment works, at no cost / risk, and now they need to somehow contribute otherwise to build your own and let antitrust law break all the proprietary channels into one common later (and risk your capital).

Yes, Tesla could allow other cars to charge but require them to have a Tesla-to-standard adapter and an account. But there is another side effect: At a Supercharger, you meet other Tesla owners, i.e. quite wealthy people, and you are surrounded by the brand. Diluting that would be like seeing a McDonalds with people who eat KFC. Tesla may want to own the visual space.