The primary consideration of the art and finery in the Vatican is to lead the minds of those who see it to God. No action is allowed that distances those things from this purpose.
To that end, note the following:
1. Much of the art was given by the artists on the condition that it would be displayed to believers, and as such the Vatican is not at liberty to sell it.
2. To whom would they sell it? That stuff is not meant to sit collecting dust in a private collection, and anyone who would buy it should immediately re-donate it back to where he got it, so you end up as you started.
Your criticism is exactly that to which the Pope is referring when he spoke against “Poverty for poverty’s sake”. You are thinking that the only value of a thing is what someone will pay for it, when the real value is what it means as reminders of the Kingdom of God. Not that you are alone. There are many people who know the cost of a thing without knowing the value of a thing.