There's no call for changing the current arrangements as:
Many people have driving licenses that have photographs that are imported from their passports. That means that it a thrid generation picture and often of insufficient quality for (relatively) frequent identity verification.
Just five figures from the whole licence number would not be unique, - it would have less than 10,000 unique combinations, and not necessarily those that use them as season ticket photocards. That's why the number is longer on licences.
The photocard should be carried with the season ticket whenever it is used but a driving licence may not.
One photocard is needed for many season tickets and it only weighs a few grams so there is no problem carrying it.
I doubt that the railway wants to have a multiplitcity of ID methods. The photcard have worked perfectly well for many decades.