It's really not that big of a cost, anyway. If the former President wants it, he should get it.
Why not give the President a larger pension and let them decide if they want to hire security or not? You could even let them hire the Secret Service. But I'm guessing they wouldn't hire anywhere near the same level of security if it was their own money to allocate.
About the only reason I can think of for why they shouldn't have this choice is that it might change the incentives of Presidents while in office, being selfishly concerned with maximising their own security by avoiding making enemies. But then again, this might be more of a feature than a bug if you think that the US already makes too many enemies.