Intelligence in the software, not in each camera
The distinction decides what an upgrade costs and how long it stays current.
Per-camera AI ages badly
When the model lives inside the camera, improving it means buying the camera again. A three-year-old smart camera runs a three-year-old model and will keep doing so until it is replaced.
Software covers the whole estate at once
Detection running behind the recorder applies to every stream it can reach, so a mixed estate of old and new cameras gets the same capability on the same day.
The camera's job is the view
What an existing camera needs to contribute is an adequate, well-placed view of the area you care about. That is a placement question, and placement can usually be corrected far more cheaply than hardware can be replaced.
Some things genuinely do need the right camera
Face recognition and plate reading have real pixel requirements, and a camera mounted high for wide coverage will not deliver them. Honest scoping says which cameras qualify for what before you buy, rather than after.