I think routes put up as mixed should remain as such. There is a disturbing trend for people to retro mixed routes. While u can not clip the bolt it is a totally different headspace and feels like cheating as you know the bolt is there. The climb loses a lot. I was at York recently and noticed that Ablutions had 2 bolts added. The original has been lifted by 10 cm so shorter people can not clip it as easily as the prior placement and so a bolt has been added below and a 3 rd near the 2 wire placements .
I applaud the replacement of old bolts but don't add more. If you need the extra bolts climb something else ie go back to Shipley etc. leave climbs as they have been enjoyed by many over the years already or pretty soon we will be like Europe with everything bolted. For those who don't know how to place trad - learn as it is so much more satisfying