Can't see the morality of euthanising fish just because you are bored with them or they are getting old and less entertaining. (I wasn't planning to flush Grandma. Though come to think of it, she is very entertaining).
Give them to the lfs for free if you can't give them a home.
However, what nobody has mentioned so far is that swordtails are quite fast swimmers and like to get around, so a 10 gallon is really on the small side. Platies who like to bumble along would be much happier.
The tank is also on the small side for zebra danios, so this might be a reason for rehoming them. Perhaps their pace has slowed because they've discovered they can't really go far. If their only problem is old age, then I would say you had a moral responsibility towards them, as they are your fish and less likely to thrive on rehoming. On the other hand, if they are not happy where they are, then it is better to find them a new home.