Some paintings in religion are done to show mystic spiritual experiences that words cannot describe and are done by holy people. Their worth is not monetary.
The churches started with the Apostles going into the world, thus many churches. "Christianity" is, in one aspect, a way to protect from persecutors. The church is holy and has power from heaven, so not to worry about discord.
Saying we only want to use the Bible is like saying we only want to read a book that grandpa wrote, but we never want to see or hear from him, grandma, our aunts and uncles or our cousins, ever again. Kinda silly.
St. Peter has the authority from Jesus. Excommunication is in the Bible when it says that we should make up with our brother on the way to the altar, or leave the altar to make up. It isn't intended to be forever or send you to another church. The catholic church believes that confirmed catholics are common priests.
Vatican City is about the size of a University. Relics are not to be sold. The church does give to the poor and churches takes offerings. We exist because Jesus is risen.