It may seem redundant since a panel layout selection is in mate-tweak but for the sake of familiarity, instead of nuking the present default from orbit and replacing it with Pantheon I believe more users would welcome the opportunity to select which system they’re coming from, so that new users receive a default look and feel from another product they’re most familiar with.
To prevent this choice from being presented multiple times in Welcome, defaulting to the panel selection screen should only happen when the welcome app detects an end-user hadn’t made a selection for this.