A longer contract increases Chelsea's risk at a certain point.
UEFA's plan is to add a rule within FFP that will only allow transfer fees to be spread over 5 years for accounting purposes. So if you offer an 8 year contract, the transfer fee can only be spread over the first 5 years for FFP. The last 3 will have £0 transfer value attributed in the accounts.
Enzo as an example, if he flops, Chelsea are tied into his wages for 8+ years anyway.