You get the best performance from a chain with the least HP lost with easy changes is why the race bikes run chains. It takes more HP to spin a shaft,they push down on the asphalt and add weight. If I had a CBR1000RR I would want a chain. I don't, I have 1/2 that HP and 100lbs more weight on my "Cruiser".
Why do BMW and the Rocket 3 have shafts? What a stupid thing to do?
They should have chains. BMW has been around how long now? Shaft is old school too.
Why do the most advanced bikes in the world from Japan Inc. have shafts on their cruisers? Because chains suck
Dirty and loud
The happy medium is belt. As above, his Sportster could be changed in his garage with a bit of money. They are a fraction of the weight of a shaft and transfer power with less HP needed. They are quiet and clean.
The average bike owner either would not or could not change the chain out on the side of the road anyhow. Today the bike dealers do a great business just doing oil and tire changes for the helpless consumers.
