So you do get it. You have just described how evolution works. So where is the problem?
Oh, it’s that you think there must be a degenerative effect. So a butterfly ‘loses’ its bright colour and only ‘gains’ a dull one. Because you think bright colours are pretty and therefore ‘good’ and dull colours are boring and therefore ‘bad’. So losing colour is a negative effect. Hmmm.
I think your mind set is pretty much like the old dear that used to teach Hitchens when he was a small boy. She told his class that grass was green because God made it that colour so it was restful to our eyes. Cue snickering from small boys who, whilst not knowing exactly why grass was green, already realised at a very young age that the old dear was so far off the mark as to be held in ridicule.
Growing more fur is a negative effect (unless the environment is getting colder). Growing less fur is a negative (unless the environment is getting warmer). Growing a longer neck is a disadvantage (unless all the leaves you need are at high level). Growing a shorter neck is a disadvantage (unless you need to crop grass). Becoming short sighted is a disadvantage (unless you need better sight at long distance to catch your prey). Becoming long sighted is a disadvantage (unless everything you need to eat is up close).
Need one go on?