You’ve not split the sentence correctly. The expression you are interested in here is on ne peut plus, meaning extremely, absolutely (literally one can’t be more). It’s a way to emphasize the following adjective, amoureux.

Have a look at the Wiktionnaire for extra examples.

La phrase, malheureusement, est mal tournée. Je suppose qu’elle signifie:

“S’il est vrai que le prestige de la culture française est en baisse / diminue en Roumanie, en revanche les passionnés de la littérature française n’en sont que plus amoureux.”

You can’t say that “prestige”, an abstract notion, is losing speed, which is mechanical.

The first formulation would suggest that because some … there is a reaction from others in favor of ….