Time to bump it up to a billion then.
If you raise player's release clause to insane levels, you need to raise his wages also.
So, he agrees to stay with you, unless someone pays insane amount of money.
On the other hand, you will pay him an insane wages to keep him here and to make him agree that kind of a deal.
For example, if Rakitic has a wage, let's say 4 Millions, and a release clause 40 Millions.
You can't ask him: let's sign a new deal, we will still pay you 4M, and you will agree on a 200M release clause.
Either raise his wages to 8-10M, or keep the same release clause (40M).
Players love our club, but they need to have their arms somewhat free.
Times can change.
If something looks impossible today (Neymar ever leaving Barca), it can change in 2-3-5 years.
Any player can suffer horrible, career threatening injuries or a disease (Gundogan, Abidal).
A player can change his personality and displays on the field by 180* in a short period like Ronaldinho or Adriano.
A club can hire a strange coach who doesn't want him here (Van Gaal with Rivaldo, Mourinho with 10s of players).
A club can bankrupt or get into financial crisis and needs to release a player etc.