I also wondered what a tank bag would do to paint surfaces. Actually, not so much the bag itself, but the dirt and dust that inevitably gets between the contact surfaces. Just about anything the contacts the paint is going to abrade the surface when vibration is added. How fast, is just a matter of time and grit size. The magnetic ones seem to make better sense than the strap-ons. But, vibration is a serious factor to consider.
Toward the original point...
The reality is, that if you use the bike, it is going to be subjected to bombardment from whatever is on or near the road surface. Sand, water, mud, pebbles, stones, rocks, oil, grease, tar, hamburger wrappers, battery acid, animal urine, and particles of anything else you can think of is going to make contact with a road vehicle.
You'll have to make the decision to either have a show piece that is awe inspiring to look at but rarely, if ever, gets used for it's original purpose, or just doing what you can to minimize normal wear and tear while enjoying machine to its fullest use. even now, the machine will never be new again, though it may look like it. But, it also tells a story of missing the point of it's original destiny.
So, did you buy it to save it, and does that give you the satisfaction value you seek? Or, did you buy it for use enjoyment value. It's a given, that use WILL cause deterioration that you can only minimize, not eliminate.
Cheers,