Ideally, it would be nice to be able to check a box to update all of this to a single new lead owner when you're deactivating a user. But, even if that's not available, you should be able to do all these things very easily with either bulk actions or automations. At the moment, you can reassign the lead owner, which also will update the opportunity, but you CANNOT currently update the task assignment, etc. This is VERY important, as it's a common occurrence that users are hired and eventually may quit or be fired and it NEEDS to be able to update easily.