It is true that there are many grand churches in the world, there also probably many more simple ones. However, the grand churches are not for the benefit of the Church per se but rather for the people of God and to give glory and honor to God.
As Catholics we believe that faith involves the whole person, not just our words. Beautiful art and buildings are meant to inspire us and draw us visually and emotionally into the realm of faith. For example, my parish is a beautiful 19th century church full of marble and art but its also in the middle of a relatively rough neighborhood. When the people of my neighborhood come to Mass or to pray they can escape the harsh realities of the outside for a little while and bask in the beauty of their parish which reflects to them the beauty of God and faith.
Grand churches also reflect our values. If we claim that God is the center of our lives and the most important thing then shouldn't our buildings dedicated to God reflect that? As human beings we naturally associate importance with how much effort we put into something. If we pay little attention to the house of God and prayer then what would that say about the importance we place upon those things?
Now, that's not to say that every parish must be a "grand church." Each local community simply needs to do what it can to give glory and praise to God. God sees the love that goes into worship. We should remember that many of these "grand churches" were built together by the poor and rich who freely and lovingly gave of their time, talents and treasures.
How much money the Catholic Church has is often exaggerated by legend and anti Catholicism. Individual parishes and dioceses are expected to be self sufficient in their finances. In 2010 the Vatican had an income of $326 million and ran a $13 budget surplus but in 2011 the Vatican ran a $19 million deficit. And while some of that money obviously went to maintaining church buildings and art, much of it also went to charitable causes. For comparison, Harvard University's endowment is over $30 billion. So relatively speaking, the Catholic Church is not really wealthy and no one blames museums for maintaining their art work and structures.
When I enter a "grand" church I am reminded of God's glory and majesty. I feel humbled and in awe. Such reminders can only reinforce the virtues of modesty and humility.