Mangala Sutta discusses things which are blessing or auspicious. Parabhava Sutta is what discusses what leads to one's downfall.
We understand this as explained by thee; this is the third cause of his downfall. Tell us the fourth, O Blessed One. What is the cause of his downfall?
Supporting one's parents is a blessing.
To support mother and father, to cherish wife and children, and to be engaged in peaceful occupation — this is the greatest blessing.
If you are wealthy and having the means and do not take care of your parents this lead to your downfall. This essentially is applicable to householders, though the Vinaya explicitly allows for monks to take care of their parents.
In the case of the Buddha or a monk, one leaves a householders life to seek enlistment and one is living on the charity or alms of others who support you seek liberation. In this case, you do not have the means to support your parents as a householder would do. So this does not lead to your downfall.
The Buddha searched for the Dhamma and found it. When the time was right The Buddha gave the gift of Dhamma to his parents which surpasses any material benevolence he could have given them.
In Anguttara i, 62, the
Buddha said that even if one should carry his mother on one
shoulder and his father on the other shoulder for a hundred years
serving them dutifully, one could never repay them. But if one could
incite one's parents to practise generosity and morality and
establish them in faith in the Triple Gem and wisdom, one does
repay what is due to one's parents. Among the Ten Subjects of Right
View, understanding that there are results of one's actions (kamma)
towards one's mother and father constitutes Right View. Therefore,
one should always hold them in veneration in one's thoughts, speech
TEN BASES OF MERITORIOUS ACTION