You think when you visit a gallery you're seeing the work of the great masters? Well, possibly, but you can never be sure. In many cases what's up there on the wall is some contemporary technician's idea of what the original artist intended- and about as authentic as the picture postcard in the museum shop. Restorers take extraordinary liberties. Did you know that a recent restorer changed the shape of the mouth and nose of the angel in Leonardo's Virgin Of The Rocks? Or that the anamorphic skull in Holbein's Ambassadors was repainted using a computer projection of a real skull in order to correct Holbein's "mistakes"? Or, most outrageously, that the recent restoration of Michelangelo's Sistine chapel ceiling removed a whole layer of the original paint- flattening the figures and destroying the sculptural effect Michelangelo was aiming for?
Picture restorers have always existed. Most paintings of a certain age reach us already well-botched by generation after generation of earlier meddlers. In the past the latest restorer left the earlier botching in place and worked over and round it. These days the approach is typically more radical; everything that isn't "original" is scraped or washed away and the lacunae filled up with new work. Two issues arise from this: 1. the radical scraping and washing often takes away original work that the restorer fails to recognize as such- including delicate glazes and varnishes. Once gone these can never be replaced and the work is irretrievably damaged. 2. The original work may be so badly decayed that the restorer, having removed the work of previous restorers, has to start again from scratch. A extreme case in point is The Last Supper in Milan. There's so little of that remaining that the thing you're seeing on the refectory wall is essentially a modern painting inspired by Leonardo's original. The choice here is between two types of inauthenticity- on the one hand work that has been remade over centuries- which has a history, reflecting changes in taste and connoisseurship- and on the other, work that is essentially bran new. It isn't obvious that the second is any more valuable than the first.
Unfortunately the restoration of art works- especially important ones- is sexy. It attracts corporate sponsorship and media attention, it gets curators noticed. Big museums all have their restoration departments. Works that aren't particularly crying out for a refit are refitted anyway because otherwise the skilled restorers would be standing idle. A production line exists and must be fed.
No-one is saying artworks should never be cleaned or restored. What they are saying is, please be gentle, show a little humility, if in doubt, don't.