Onions and carrots side by side is good for many reasons....they complement each other. A good row of onions for me grows to more than 60 cm thereby providing another barrier to carrot fly. The odour of onions does help confuse the carrot fly.
Fleece is by far and away the best solution to carrot fly I think. It is light, moderates temperatures and is aesthetically pleasing. Polythene gets heavy with dirt and rain so can damage young crops and barriers look awful. As soon as seed is sown cover with fleece.....it keeps off cats, etc., whilst germinating and warms the seed. Being lightweight it does not weigh down the emerging carrots. It is easy to access when pulling carrots too. I use metal U shaped pins to peg down the fleece
I agree that, along with everything in nature, pests are more savvy now.
Note that parsley too suffers from carrot fly.
I don't have a problem with carrot fly.....I use fleece early but have natural "barriers" around my veg patch, viz., a mixture of shrubs to one side, a lavendar hedge to another and a runner bean framework on another during the summer.
My veg patch is smaller now so rotation is not really practised as well as it once was but, so far, it's effective
I grow veg in rows grouped in 4.....viz., I have a tool that makes 4 rows 3" apart. Each group of 4 is separated by 15". I use this for carrots, spring onions, Beetroot, etc but onions are grown conventionally