I've discussed this topic and levied criticisms at common issues I see in this genre in an article called What is Content?. Essentially, I think it's better to have planned out the scope of the game in advance (as I've done for gamedev tree, advent incremental, and Kronos), so that the game feels like it has a proper end. It also sidesteps the issue of having new layers outright replace the earlier gameplay rather than extend it, as is common in games that keep asking themselves what to add next. Basically, planning makes better games.