Why preservation is the whole job
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A skincare product is water, oil and food for bacteria, packaged in a container the customer will put their fingers into for six months. Everything else about this craft is downstream of that sentence.
What you are actually making
An emulsion is a suspension of oil in water, warm during manufacture, nutrient rich, and at a pH close to skin. Those are, almost exactly, the conditions a microbiologist would choose to grow something.
The product is not preserved because it is clean. It is preserved because a system was designed to keep it that way after it leaves you.
The three things that must all hold
The preservation system is not an ingredient. It is water activity, the preservative blend, the pH, the chelators and the package, working together. Change any one and the others may no longer be sufficient.
Water activity above roughly 0.75 supports microbial growth, which covers almost every emulsion. pH decides which preservatives are even active. Chelators tie up the metal ions that both degrade actives and assist organisms. And the package decides how much air and how many fingers reach the product over its life.
Why this is different from food
A dry food is safe because nothing can grow. A canned food is safe because a kill step was applied and the container keeps the world out. Skincare has neither: no kill step at the end, and a container the user opens daily.
So the defense is entirely chemical and entirely designed in advance. There is no step where you make a bad batch safe.
Where most small brands fail
Not in the formula. In the assumption that a preservative listed on a supplier's page at the recommended percentage is a preservation system.
It is a component. Whether it works in your formula, at your pH, in your package, is a question with one answer, and the answer is a challenge test. Skipping it is the single commonest and most serious shortcut in this craft, and the product that fails does so in a customer's bathroom rather than on your bench.