Could work, but you'd need a behavioural specialist to figure out if it was worth it, along with some statistical analysis over when people drop subs, specially the ones that drop between expansions.
It could work for them if it keeps people subscribing, at the end of the day every person that's subscribing is a potential friend of someone who's friend got them into the game, which is another big cash drop and regular income for them.
To be worth it, they just need someone to be subscribing for 3-4 months more than they otherwise would, which isn't a huge ask, specially if they add in a 6 month commitment, you could even say the 'worth' is lower, as its reliable income for them that's easier to plan around.