Hosting your shit costs them literal pennies at absolute most. "This user owns these games and keys" is bytes of data. If they have cloud saves it's slightly more, but about 100 orders of magnitude off of affecting their bottom line in any way. Old games they do or don't support have nothing to do with inactive accounts. Active accounts own them too.
There is zero financial rationale for this. It's not anything resembling a rounding error on the budget sheet. It's basically free.