Your question seems to imply there must be some secret source, some behind the scenes force. I'm not sure what you expect to find, but money doesn't grow on trees for Catholics any more than for Protestants.
In fact, if anything, Protestants have a harder time with this than Catholics do. The question you linked to has answers that explain very clearly that Catholics are obligated to give. This sort of obligation tends to be taken much more seriously in Catholicism than in Protestant circles. Even in countries where belief is nominal, fulfillment of such obligations often outpaces other aspects of devotion. In addition, since the Catholic church is centralized, the local ups and downs of giving are averaged out and the expenses are spread out over a larger giving base.
Just because some Protestants define a tithe as a regular 10% offering doesn't mean that actually happens. In practice it is usually much much smaller than this. Since the funds aren't centralized, this often means local churches may have noticeable shortfalls at various times.
I still feel that only offerings may not be sufficient.
It doesn't matter too much what you feel, the account ledgers say otherwise. Of course they have hard times too and giving shortfalls mean less ability to carry out projects etc, but the fact of the matter is that Catholics do give and these offerings do support the church.
The Catholic church benefits from a large member base and their obligation to give out of their ability to the needs of the church. This is the major source of income for the church. End of story. There isn't some vast hidden business network or other money making scheme.