Thinning grape vines, reconsidered

Why thin?

Thinning presumably enhances the long-term health of the vine. However, I heard from a friend that the productive lifespan of St. Vincent vines is 7-10 years. Our vines (early summer, above) have been up since 2006, so what's the point of losing yield at this stage? Why not just go for it? Thinning would be like a couple in their 70's postponing a cruise until a better time later in life.

And then there's this, from Growing Grapes in Missouri, page 17, by Missouri State University-Mountain Grove:

St. Vincent is considered to be a chance hybrid of a French-American hybrid cultivar with an unknown parent. It is a red grape for wine with large berry size and small, loose clusters. It has high vigor and moderate to high degree of winter hardiness. The fruit matures late season. 

It does not require cluster thinning. 

Yield is high. The vine trains well to a cordon system with spur pruning. A good spray program is needed to control diseases. Loose clusters make it not susceptible to bunch rot. Wine quality is good. It is typically made into a dry, red wine, or used in blending.