If we are to build beautiful churches it will cost money. Lots of money. The pragmatists look at the price tag and pull back. “Surely we can make some cut backs!” “Surely we should be giving this money to the poor.” “Surely we don’t need quite such a big church or quite such expensive fittings.”
There are a couple of considerations. First, the question of building beautiful churches for posterity or giving to the poor should not be either/or. We should be generous to build beautiful churches and give to the poor. 
Second, we should look at our income and the income of the Catholics in the past who did build beautiful churches. Most of us have far, far more disposable income than our grandparents and great grandparents. They are the Catholics who built the big beautiful churches, convents, monasteries and schools in the big Catholic cities in the Northern states. 
The most important reason Catholics gave generously in the past is that they really believed they were not only building a beautiful church for posterity, but they believed by doing so they were investing not only in their grandchildren’s future, but in their own. In other words, there was an eternal dividend for generous giving here. When you gave a stained glass window or a beautiful altar, or a wonderful tapestry or whatever else, you trusted that whenever that window was gazed at, that chalice or ciborium or vestment or altar was used for the holy sacrifice, that you would be prayed for, and masses would be offered, and your offering would be part of the greater offering of the whole church for the good of your soul, the salvation of the world and the good of all of Christ’s church.
Your donation of a material gift had eternal consequences for you and for others. This is what we’ve lost, and this is the main reason why Catholics don’t give as generously as they once did.
Quite simply, they don’t believe it matters.