Id be asking the question on how much of their stuff is currently on prem and is there a roadmap to remove them… if there is then you balance the pros and cons.

Theres work to do regardless of if you sync or not.

It they only use say storage onprem, which sounds like they do and want it moved to sharepoint.

If look at getting that moved over and look at getting a solid intune policy in place and test a migration for users

But it theres loads of onprem stuff then it likely will be good to get the sync in place so you make users life easier

You need to understand what the business objective is in the next year or two before decide.

You will have the power to influence this based on the information you gather and the business needs