Make previous parts of the campaign insignificant.

[Rinin]

I mean, you guys already solved the problem one time in old campaign. Let the player build small base, then expand the map to the bigger objectives, and let him keep the old base as suplimentary. Just like in real game.

If campaign is partly tutorial - player have to rebuild bases, because first one will be spagetti, and forsing player to keep it is actually bad thing.

You don't have to remove old bases, just make them unsignificant to new tasks. Players should be able to use them, not forced to. Also players should be able to rebuild it on base 2 if they already learned some mechanics more and want perfect pretty square base. But again, they shouldn't be forced to rebuild.

You may give some limited expansion capabilities to bases. Like conversion of base 1 to main power plant, base 2 to be huge smelter center, and so on, but don't expect player to make proper red/green module from get go. Otherwise your future plan for megabase will have bad foundation.

Make it possible to start from scratch, but don't force it. It should be enough space and metall on base 2 to repeat base 1 in cleaner way and so on.

Eventually you will have to stop. But make several first bases insignificant and easy to rebuild. Just to make some room for learning from mistakes.