A dirty rear derailleur of course also reduces the drive train's efficiency by a lot - which can be solved by cleaning the chain every other month.
When moving towards belt drives, you need a very stiff frame which can be opened/split in the back to fit the belt. These frames are more expensive to produce which will furthermore increase the overall bike's price.
Ideally, we all train our legs to be able to handle a single speed setup ;-)
When it comes to efficiency, internal gear hubs sadly aren't yet in the range of a rear derailleur.
https://fahrradzukunft.de/17/wirkungsgradmessungen-an-nabens...
A dirty rear derailleur of course also reduces the drive train's efficiency by a lot - which can be solved by cleaning the chain every other month.
When moving towards belt drives, you need a very stiff frame which can be opened/split in the back to fit the belt. These frames are more expensive to produce which will furthermore increase the overall bike's price.
Ideally, we all train our legs to be able to handle a single speed setup ;-)