The problem is that this proposal is based on several false premises. The JWs do not offer a "comfort zone" in any sense of the term. In fact, being an active JW requires a constant state of discomfort, excessive demands on time, no holiday celebrations, public humiliation in the "field service".....if a DFs ever does return, it is solely due to family reunification issues, and rarely the result of some existential crisis about the eternal salvation in paradise crap.
Second, the idea of return has to consider the different TYPES of situations that causes someone to leave. For example, I was a "born in" that faded out before ever being baptized. It was never a comfort zone for me, I was just never even remotely interested in it. Indeed, I have never known a "born in" non-baptized fader that went back. In the other categories, we have the DFs, which I think fall into 2 broad categories. 1) Genuine believers who gave into temptation (99% of the time its sex) and 2) "Rebel" JWs who got "caught" and are DFd. I would propose that only category 1 would ever return, because they are the only ones who ever believed any of it in the first place.
All in all, I would estimate the percentage of those leaving (for whatever reason) and eventually coming back (permanently) would be something on the order of 0.5 %. Of those, many of the "reinstated" will just get DFd again within a year or two anyways.