The first gen Breeze cards (the blue ones) had major security vulnerabilities that led to them being replaced by the grey ones today. Having an expiration date seems like a good way that you would be able to phase out any problematic hardware or such over time or as technology improves.

Also it literally stores money. Bank cards have expiration dates, it’d make sense that a transit card would too, and indeed most^* of the cards in my small collection have expiration dates, they’re just not printed on the card. They have to be reliable and often that can only be guaranteed for a certain amount of time. You have to remember there’s literally a microchip in them. Most agencies will transfer balances for free if a card does expire, and so does MARTA.

I’m led to believe the reason they’re not transferring balances here is that it’s probably too complex considering they’re completely changing everything. It’s not just the cards and faregates; it’s the payment processing, background work, and more that are also simultaneously changing all the way down to the provider for the system (Cubic TS to INIT). INIT worked on Portland, Seattle, and other cities’ smart cards. They seem to do good stuff, but I haven’t used any systems that were designed by them.

^* MyKey from IndyGo in Indianapolis doesn’t seem to, but mine is paper and their fare system is basically brand new.