Fully sealed drive train was produced by Sunbeam 100 years ago. Chain ran in an oil bath. Sunbeams were the most expensive English bike made. Many originals still exist, a few remain in use, the oil bath chaincases are still tight. The famous statue of Sir Richard Elgar with his bicycle - that is his Sunbeam.
Only problem with the Sunbeam was removing rear wheel to change a flat. Of course any who owned one had a mechanic. It was a lot of work. Solution for that would be an arrangement like the Cinelli Bivalent hub. When you opened the quick release on a Cinelli hub and dropped the back wheel the entire drivetrain remained in place. It would not be that difficult to go from there to an enclosed drivetrain on a fairly normal useable bike.
Another feature that would make enclosed drive more useful would be chain and sprockets that did not wear out. Of course simply being enclosed would limit wear considerably. Next step would be reduced chain pitch. Coventry Chain produced 8mm pitch ‘Chainette’ from 1909 to 1939. Thousands of races were won on that chain, most notably by Freddie Grubb and Sir Hubert Oppermann. When Shimano tried this some sixty years later they touted stiffness and completely missed the idea. Coventry used chainwheels and sprockets with a far higher tooth count, which meant smoother operation and far less load and less wear on each tooth. There are a variety of chain designs more efficient and less prone to wear than current bicycle chains but all of them require minimum sprocket tooth count higher than now used by bicycles. With shorter pitch chain the required tooth count would be simple. For most users the result would be chain and sprockets that lasted the life of the bicycle.
There are lots of ways to improve bicycles that are entirely overlooked. Innovations come from the sales department, not from engineers. Customers are easily hypnotized by advertising and accept rubbish that should have been discarded long ago, celebrate novelties of no particular benefit.