I am aware of the drawbacks mentioned in MN.19 , but could this way of thinking, that is, the shortness of pleasure not lead to isolation/life rejection?
Yes. MN 19, similar to AN 3.38, MN 26, etc, describes the experiences of the Buddha-To-Be (Bodhisatta) before enlightenment. For the unenlightened Gotama, his experience of life was happiness was not to be found in ordinary worldly life; thus he went searching. While Gotama did not know what Nibbana was; he already held the conviction that ordinary worldly life would not bring him true happiness.
If we ourselves personally have not had such an experience of disenchantment towards worldly life then teachings such as MN 19 may not be suitable for us or, otherwise, may need to be adapted into a reflection that focuses more on morality rather than on Nibbana.
The Buddha frequently used the word dispassion towards the world, but I HIGHLY DOUBT HE HAD AVERSION to worldly things. This is a common
phenomenom that many monks also display ,that is, that they feel
aversion to the world. How does one deal with such a (mind) problem to
not to drift into negativity to worldly affairs?
There are subtle differences between aversion/hatred (dosa), disenchantment (revulsion; nibbida) and dispassion (viraga). Therefore, impressions can arise, which can be actually true, that monks can have aversion & hatred towards worldly things.
Aversion is an ordinary, natural yet unenlightened reaction towards evil worldly phenomena. For example, most people believe Hitler was evil, therefore they have a hatred towards Hitler. Thus, generally aversion occurs within some kind of moral outlook related to ideas of 'good' & 'evil'.
Where as dispassion, which arises from disenchantment, is related to impermanence (anicca) and the inability of impermanent things to bring lasting & true happiness (dukkha), as described in the Buddha's 2nd Sermon (SN 22.59). Dispassion is essentially a loss of craving that occurs with the view that worldly things cannot bring true happiness. Thus worldly things become unattractive & not worth craving.