You're right. Worms weaken the immune system and eventually making birds susceptible to all kinds of problems.
Also, one female large roundworm lays thousands of eggs a day onto the soil. Here's a link about worming chickens, very good info...
If birds arn't on soil, they cant get worms. Alot of folks raise birds on wire, so they dont really have to worry about their birds picking up worm eggs off the ground. That's not to say that an infected insect might wander in, but not likely having ingested worm eggs, only time would tell after...
It's a FACT that birds kept on the same soil are more susceptible getting worms and NOT by free ranging.
Ivermectin has lost its effectiveness treating roundworms in poultry, due to its overuse treating external parasites on poultry.
The only way to tell if birds have worms is having fecal samples looked at under microscope.
There are alot of reasons why chickens can become skin and bones, for example; ecoli infections, enteritis, impacted gizzard or crop, sour crop as you mentioned, tumors/cysts, wasting away, as well as...
You are correct about large roundworms having a direct lifecycle and tapeworms having an indirect lifecycle. However the following poultry roundworms have both a direct and indirect lifecycle:
Capillary worms, cecal worms, eyeworms, gapeworms, gizzard worms.
It's also true that not every insect...