The problem with that is that you just get better and better at the thing you do all the time. Specialisation to the extreme is probably most optimal for society as a whole when there are enough people.
Societies' gains as an effect of this specialisation is just one (albeit an important one) aspect of the whole optimisation.
Anti-fragility and redundancy typically decrease the number of (costly) disruptions in a system and thus the proposed fallback profession might actually be a net positive for efficiency rather than a loss.