I wouldn't blame JT for the failures under Guidry, but why anyone here assumes JT is ready or capable of calling a top 10 defense aside from being a HOF DE is beyond me. One doesn't mean the other is a sure thing.
I wouldn't feel comfortable handing over a top 5/10 defense to a novice play caller, and if we did I wouldn't expect the results to be similar or close to what Hetherman has produced this year.
If Hetherman leaves, then you go out and hire another stud DC who has experience.
The only caveat here would be if JT has been instrumental in the defensive scheme and game planning this season, and has been a primary reason for Hetherman's schematic success. However, I don't think that is likely the case.
The next phase of the Miami program under Mario has to be the development of staff personnel. With success comes staff departures (usually hired to better gigs - head coaching spots, NFL, etc.), and to hedge against this volatile turnover, we need to make sure we have personnel ready (if needed) to step up and produce.