Against
Opponents of retro-bolting cite the adventure aspect of climbing and respect for the FA as reasons for not going ahead.
Climbing is an adventure sport that is inherently risky. A climber should understand and accept this risk. If the FA is exceptionally skilled and put up a climb that was both difficult and dangerous, then that is to be respected. The climb should not be "brought down to your level" by the addition of extra bolts which will ruin the adventure aspect of the climb for someone who is proficient enough to complete the climb in its original state.
If a route is climbed very often then it can quickly become polished. This is where the rock becomes smooth as the friction of people shoes and hands wears it away. This can be seen in clear evidence on well travelled routes such as "Saul's Crack" (HVS 5a) on the Upper Tier of The Roaches, Peak District, UK. The retro bolting of a route makes it accessible to more climbers which will result in the loss of quality of the rock through polishing.
The first ascent of a bolt-free climb will have caused no deliberate damage. For subsequent climbers to then deliberately drill that piece of rock; cliff or crag to placing expansion bolts, to bring the grade of the climb down to their level, is unacceptable.
Read more about this topic: Retro-bolting, Argument