I'm not too sure if you're thinking of competition from other companies or from individuals producing their own parts, but it doesn't make too much difference really. Just because the designs are open source doesn't mean everyone will prefer to make the parts than buy them.
- Hobby 3D printers and CNC milling are of much lesser quality than professional/industrial machines
- Even if you were happy with lesser quality, such machines require a lot of specific knowledge that most end users of the parts won't have and won't be willing to spend the time learning
- And there is a large upfront cost in these machines. Whether it's open source or not, larger volumes scale better
If your business has the equipment, trained personnel, good quality assurance processes and a head start, it's likely that you'll be able to maintain a profitable business for some time. As an example, I think Tesla were talking about licensing their batteries etc. for free (this involves patents rather than copyright, but it's still a decent parallel). So sure, anyone could make those components. But there will not be many people who could start a successful competitor to Tesla before those components are obsolete, and the size of the market is so large that those who do will be very unlikely to hurt Tesla at all.
Opening your hardware designs ensures that if your business goes bust that the innovations you have will not be lost, and it allows them to be brought now to markets you can't compete in. If the international shipping costs for your product are prohibitive but you have no desire to manage many international production factories, then opening your designs allows others to start producing the products for the international market so that more people can benefit from your amazing product.