That was old days - nowadays License model is based on usage-based. (so that will give the flexibility you can buy only the License required for the business) rather buy UL cost and you do not use it.
Look at the below :
Universal vs. Specific Permanent License Reservation
There are two separate types of Permanent License Reservations:
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Universal Permanent License Reservation (Universal PLR or UPLR)—The Universal Permanent License permits perpetual, unlimited use of supported firewall products, including all optional licenses. Once you purchase and apply a Universal Permanent License, any applied feature licenses, which are normally time-based, are permanently applicable. However, you are still responsible for purchasing replacement licenses as they expire in your Smart License account. ISA 3000 does not support Universal PLR.
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Specific Permanent License Reservation (Specific PLR or SPLR)—Specific Permanent License Reservation requires the same number and types of licenses as standard Smart Licensing. When you obtain this license, you select which optional feature licenses you want in addition to the base license. You must periodically update your licenses as they expire.
https://www.cisco.com/c/en/us/td/docs/security/firepower/660/fdm/fptd-fdm-config-guide-660/fptd-fdm-license.html
https://www.cisco.com/c/en/us/td/docs/security/firepower/licensing/faq/firepower-license-FAQ.html
