that one would study medicine for five to seven years with the intention of keeping it as a back up.
One well-known chap in Melbourne (certainly would have to be retired now - haven't spoken with him for quite a few years) ..
(a) started off as an airline F/O (ANA - long time ago, now),
(b) read medicine while flying, eventually discontinued airline flying and ran FT with medicine - but could have combined the two without too much difficulty,
(c) subsequently read electrical engineering and ran a successful avionics shop
all the while having an interesting life of it. Not the common path due to the training time, but feasible, nonetheless.
One of the PPRuNe moderators combines both medical practice and professional flying. I venture to guess that there will be others in the sandpit as well.
As with any of the vocational degrees, they become very stale, very quickly if one doesn't work in the field along the way - none of them really fit the idea of being kept in reserve for a feared event years down the track. I see no reason why most can't be combined with flying. For instance, I ran a very active aviation engineering consultancy activity in parallel with airline flying.
I guess that similar observations could be made regarding, at least, some trades which require licences ?