There is no need to restart every Alpha.
For many years now, including every Alpha that I've played, the devs have made the previous Alphas available to re-download at any time in the game's Steam settings (instructions are in the pinned FAQ). You can set your game to download Alpha XYZ, and stay on that Alpha indefinitely no matter what happens in the future. If you are happy with Alpha XYZ then you can completely tune out that the game is still in development, if that's what you want. They've been doing it for so long that there's no particular reason to think this practice won't continue.
If on the other hand you think "there's no fun there," then... good news! The developers agree with you. That's why they kept working on the game, instead of stopping at Alpha XYZ. If you stop to think about it for a bit, it wouldn't be feasible to make worlds made in the old Alphas compatible with newer versions. You would have chests full of sharpened sticks and adobe ramp...
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