You don't need a highly paid accountant nor lawyer to understand basic accounting rules. Assets can be written off over the lifetime of their asset. There is nothing wrong nor illegal nor surprising with what Chelsea is doing. It is not a loophole, it is how amortisation works. A loophole and highly paid lawyers and accountants do things like sell the ground/stadium to themselves and register the money as profit, albeit this loophole has now been closed. From a financial perspective it is more prudent to have shorter contracts as the company is just pushing more debt to future accounting windows with longer ones. Clearly, Chelsea don't have as much FFP headroom as they would like hence the longer contracts. There is nothing more cynical in doing this as there is getting a player on loan, why should a club be allowed a player of higher value for a small loan fee and partial wage payments?
In the UK, the clubs are strictly only permitted to determine the lifetime of the asset (player) based on the duration of the contract, this is why clubs renew player's contracts early during their contract as it reduces the annual amortisation cost.
Barcelon's accounting practice has long been to amortise their payers for periods longer than their contract, this was / is permissible in the Spanish league. So they get the benefits of lower annual payments without the detriment of having to commit to wages for a prolonged period of time. This is not allowed in the UK and this is one of the reasons Derby Co failed FFP and got points deduced as they were amortising for periods longer than the player's contract term.