Bath tub supports

There would be no such thing as "excessive wear" on a bathtub drain - unless there was caustic material being poured down the drain which ate away at the drain pipe and joints.....although from the sounds of it this is far from the case here.

For the sake of discussion let us assume that the tub is a quality name brand product.

It may be that there was excessive flex on the drain pipe joint to the tub, especially in the case of a plastic or fiberglass type of tub. Excessive flex would only be brought on by an improper installation of the tub where it was not supported in the correct manner or the drain pipe was improperly installed.

If you clean the old caulk off, properly prepare the joint, use 100% silicone caulk ,and your tub or walls do not move more than they are suppose to (natural flexing is ok), then you should have no problems with water at the caulked joint.

For best results, fill the tub with water before caulking, and keep the tub filled until the caulk is completely dry. This will open the joint as much as possible and then leave it under compression.

If it is flexing too much and that is breaking the caulk (you mentioned it but 100% silicone should be able to flex a lot), then maybe remove the tub and reinstall with a proper mortar base. If it still moves too much is the floor structurally deficient?