Well, if your labour is not, in fact, fungible it won't behave on the market as if it is fungible. You'll see this often for people paid a lot doing a single role for 30 years. Given the probability of a good replacement hire, they can't actually be replaced. Given their unique knowledge of that company, they're worth 50% to another. You'll also see this at startups where people will move from place to place keeping their income. But the median Facebook engineer's labour is fungible. It doesn't really matter which engineer is there so long as there is an engineer there.
And human beings are not being treated as disposable gadgets. Their labour is behaving like any other good in the market. There are some specifics but for the most part it is true.
There's no should to this. It's merely observational truth.
I think it is a mistake to conflate the intrinsic value of a human with their point-in-time value as labor in a business relationship. They are very different things.
In a business relationship both parties need to find a price point that is mutually satisfying. If there is no agreement that doesn't mean that either party is treating the other like "disposable gadgets".
And the system is not static, value from both parties perspective changes over time, which means the mutually satisfactory price point can also change or even evaporate.