Can a thermal jersey replace a winter jacket?

In moderate cold (3–12°C), yes — a thermal jersey often eliminates the need for a jacket, which is why it's such a popular piece. It breathes better than a jacket and is more comfortable during hard efforts. However, in temperatures below 0°C, in rain, or in very strong wind, you'll still need a windproof or waterproof shell over the top. Think of a thermal jersey as your primary insulation layer, with a shell as your weather protection layer when conditions demand it.