I grant that this is a good objection to the doctrine of the rapture. Considering the fact that doctrines such as the Trinity, salvation, etc, were all decided a long time ago, it would be problematic to suddenly come up with a new idea.
In order to present a sort of defense on this issue, I offer the following thoughts:
1. The concept of Sola Scriptura would allow for a new concept to become doctrine regardless of when it came to be. While sola scriptura mostly surrounds the issue of salvation, it can also infer any doctrine we discuss. If we are to assume the tradition alone dictates whether we believe something in the Bible are not, then we have missed the point of sola scriptura. Besides, if a person requires that a doctrine be around for a significant period of time before is acceptable, then he or she must decide what the line of demarcation. Furthermore, should the Lord tarry for another thousand years, would that suddenly make the doctrine of the rapture more relevant? If 200 years is not long enough for doctrine to be acceptable, how about 1200 years? As you can see, establishing a line in the sand regarding church tradition as it relates to the Bible doctrines is a tricky business indeed.
2. If the doctrine of the rapture is completely baseless, I submit that it would never have become popular in the first place. Are we to assume that God is so powerless that he cannot prevent bad doctrines from coming into form? I would only assume that it would be a case of cultic activity that the doctrine of the rapture would've come into being outside of God's will. Doing so would equate the doctrine of the rapture with cultic device. Do we really want to equate believers in the rapture with Mormons, Jehovah's Witnesses, etc.? I certainly hope not!
As you can see, the relative youth of the doctrine of the rapture is not necessarily enough to force it's rejection. That must be done through Scripture interpretation and common sense alone in accordance with the concept of sola scriptura. I believe that there's evidence in the Bible of a rapture event, and believe that I can prove it as time goes on.