Welcome to the forum Ignasi, and good question.

It's something that happens to all of us at some point. Manufacturers just didn't expect their watches to be in use as long as some collectors have them and, even where they make a virtue of longevity (like Patek Phillippe's advertising strapline), the luminescent material is always going to be a weak point. In some cases, the radioactive activator loses activity because of short half-lives, or the lume material itself (a mix of adhesive, a fluorescent/luminescent material and something to excite or activate that material) absorbs small amounts of moisture, expands, cracks and eventually crumbles away.

Your watchmaker does indeed sound like a serious guy. Some will attempt a relume and do an ok job, but the worst relumes can be completely disfiguring. Where the lume material is added directly to the dial, it is then often very difficult to remove without in turn damaging the dial.

Personally, I take it as a fact of life that lume will eventually fail and leave the dial alone and original when it does, but other viewpoints vary - you can search here for "relume" and you'll see a cross-section of what those view are, as well as some of the horror stories that have resulted! A good start would be:

http://www.chronocentric.com/forums/heuer/index.cgi?page=1;md=read;id=41584

although the picture in the original post is no longer of the watch in question, so don't go by that!