I see your point but I don't think it's correct. The fact is you are second choice (at least), along with every other guild that isn't leading progress on the group of realms that the player is free to raid on. Just because they settle for you doesn't mean that they won't stay for years to come; that's down to the experience that you and other officers/guildies provide. Of course there are people who deliberately join a guild with the intention of better dealing you later, but there's no way you can tell that from their app unless they lack the intelligence to deceive.
In my experience though, the vast majority of players only leave because the guild is no longer (if it ever did) provide for their gaming needs. For this reason, I think you do better to quickly find out what their needs are during the app/trial process and honestly weigh up the chances that your guild is a good fit for them. In other words, don't do what a previous guild of mine did and say they offer a stable environment when they recycle 50% of their raid roster every month.