Why not trust that the Ribbono shel olam created a zivug for every girl instead of being over on an issur dirabanan out of desperation?
Why is that a contradiction? There is no concern that there is no zivug. There is a concern that people set standards which will result in unmarried girls. I take 2 things from the Tosafos. 1) Shidduchim is so important, that we risk being oiver an issue d'rabanan to make sure girls are married. 2) Societal standards are so important that they can change the course of accepted norms regarding halacha. This speaks the importance of setting and regulating communal standards and combating any craziness which may set in.
(Tosafos is also not a history book. While they may note the custom to ignore the issur as a result of the prevailing standards at the time, they don't document what steps were taken on a societal level to set reasonable standards or to change them over time. I don't know enough about the era to comment on whether the standards were reasonable or what debates or opinions were posed by leaders at the time for or against the status quo.)