I don't particularly believe this, but it fits Occam's razor, so it seems to deserve some examination.
Also this "more fragile people" argument assumes the "fragility" is both inherent and of a lifelong kind. This ignores that most causes of infant mortility are external, and that for many of those being exposed to them results in a lifelong increased mortality risk. Excessive hygiene leading to more allergies is a direct example of this.
How? You can use that to decide between two (or more) explanations, but you only presented one.
For clarity, I will include both here:
The two explanations for increased adult fragility are:
forgotoldacc> Parents shelter their children too much and have created adults that have additional allergies as a result of lack of childhood exposure
rocqua> Increased sheltering of children has allowed more of the fragile ones to survive to adulthood, increasing the number of fragile adults we observe today.
A lot of people who today would be considered to have a condition which is entirely treatable by doing (a), taking (b), not doing/avoiding (c), etc, would, a century ago, have just been kind of deemed broken. Coeliac disease is a particularly obvious example; it was known that there was _something_ wrong with coeliacs, but they were generally just filed under the 'sickly' label, lived badly and died young.
(And it generally just gets worse the further you go back; in many parts of the world vitamin deficiency diseases were just _normal_ til the 20th century, say).