Well I actually can substantiate it by the fact that the children of previous Swedish princesses took the status of their fathers when their fathers were not royal.
Those princesses weren't in line to the throne and neither were their children, so there situation is not comparable. If Madeleine had not agreed to allow her children titles (which was most likely the wish of her father - Leonore was born as the spare in her generation!), they would have been the first in Swedish history to be in line to the throne but not be prince(ss).
Imho the alternative of having non-royals in line to the throne would have been option as well (Norway for example chose that route - but Märtha Louise herself had already been demoted, unlike Madeleine) but the situation of her children was a completely new one to Sweden. Children of a princess in line to the throne; the only other person that applied to was: Estelle. Of course, she was in a special position of being also the heir to the heir, so, they could have decided either way but there wasn't a precedent to follow.
Currently, the European monarchies are divided in whether everyone in line to the throne is titled or not.
Only titled:
Sweden (all princes; some HRH and 'of Sweden'; others no (longer) HRH and of Sweden)
Denmark (all princes; some HRH; some HH)
The Netherlands (some princes of the Netherlands (HRH); some counts of Orange-Nassau; previously: some princes of Orange-Nassau (HH))
Belgium (all princes - all HRH)
Luxembourg (all princes - some HRH other HH - some of Luxembourg; some of Nassau)
Liechtenstein (all princes - all HSH)
Both titled and untitled:
Norway (children of ML (a princess but not a RH) are untitled)
Spain (children of infanta's are grandees of Spain - sons go before daughters in line of succession)
United Kingdom (children of princesses are untitled unless their father is titled/received a title; children of non-titled people are also untitled)
Monaco (children of the Sovereign are titled; the others are not)