The idea is that by designing a new platform you can create one which has a 10-20 year lifespan. These platforms tend to be designed for both electric and hybrid cars, can have variable wheelspans, tracks, and widths while retaining a common set of drivetrains. The MQB and MLB platforms from VW are two examples of this.

Monkey patching an old platform might get you another model generation, but ultimately it will end up being more expensive and less flexible in the long run.