How to motivate users to archive/delete to improve the performance of the system?

Users on our platform can create lists of groups that have unique items. The number of items in these lists can reach millions.

Over time users have created hundreds of these lists to use in various ways on our platform. The calculation of these lists puts a strain on our system.

Recently we implemented archive functionality that archives/deactivated selected lists. But the clients are very reluctant to use them. Especially some of the clients that have thousands of these lists don't bother to archive hundreds of their lists which is understandable.

It is clear that the majority of users have created a lot of lists in their account that they don’t even use and have no incentive or inclination to remove or delete.

There are a few solutions that come to mind, they aren't perfect, but we have to solve this issue sooner than later.

a) Hard cap the number of lists that can be created

b) Automatically archive lists depending on some criteria (e.g. last time used)

c) Periodically propose to the user to archive lists (e.g. "we noticed you have 124 unused lists, do you want us to archive them?" )

I'll be happy to hear your ideas and thoughts.