I think it's kinda impossible to separate out the environmental conditions from the difficulty. Maybe it would be 8C/+ in pristine conditions, but short of someone inventing teleportation it's never going to get climbed in those conditions so guessing a grade for those theoretical perfect conditions doesn't make sense.
Yeah it's a tricky one as there's no point grading for conditions that never exist, but how do you try and take into account both the best possible conditions and/or how rare they are/how different they are to average conditions? What if the best conditions only happen once every 10 years and/or you need to spend all winter shovelling snow off the top to get it in good nick?
IMO the grade of a thing only make sense in relation to the style in which that thing is usually done, e.g. if a route predominantly gets onsighted by people on short holiday trips and is rarely if ever sieged, it probably has a grade that reflects how hard it is to onsight on holiday in medium/ok conditions. A potential 9A boulder is generally going to be sieged by full time climbers who are prepared to invest long stretches of time into it, so maybe the grade for a 9A boulder makes sense to be more reflective of the best possible conditions you're likely to see over a season rather than over a week?