I traveled cross-country with a down bag. Bag getting wet when camping was not a problem. Problem was bag drying after being washed; took too long. After that trip I switched to high grade synthetic bag. I don't plan on going back to down. My synthetic bag packs small. In looking for a synthetic bag I set weight limit at two pounds.
Never heard of washing a down bag. For fun I went to the Marmot website and looked at their synthetic bags. They have a 2 pound synthetic that is rated about 32 F for men and 42 F for women. Cloudbreak 30 model. Retail price of $169. I have a Vaude down bag with similar temperature ratings. It weighs 1 pound. Half as much. Paid about $90. It packs up to about the size of a large grapefruit. Probably 1/4 the size of the above synthetic bag.
Double the weight. Four times as much space. Not automatic deal breakers. But easily apparent disadvantages. If everything on your bike trip weighed twice as much and took four times as much space to carry, you would not be able to tour by bike. Unless you wanted full panniers and pull a BoB. And only rode 30 miles a day. Would you tour with a large 8 pound tent? Most bike tourers probably try to find a small 3-4 pound tent. Would you tour with biking shoes, hiking boots, sneakers, and thongs? Or just use your biking sandals for everything or maybe also include a set of sneakers with the biking sandals.