I think it's a fair concern. The same concern would be shared especially in the outsourcing industry, which involves companies or organizations turning part of their business out to another company to run more efficiently.
From a contracting perspective, there are ways to mitigate the risks you've just outlined in competing and migrating from one contractor to another. I've seen it happen a number of times successfully. I've seen it happen unsuccessfully, because people have not anticipated what would happen at the end of the contract and how long it would take to migrate and to consider an alternate service provider at some point in time, but I think it's feasible to address the issue you've raised through an appropriate contracting and tendering process.