Semiautomatic rifles were very new, by WWI. The only one to get into any production was the Mexican Mondragon, and only 4,000 of those were made by SIG before the Revolution cancelled Mexico's order. The Germans bought the remaining 3,500, and the design needed some development, as they did have problems in the trenches with them- they apparently did not like dirt. But adopting them would have been impossible. The Germans had about 11 million men in their army, so that would have meant about 1 Mondragon for every 3,000 soldiers...hard to make millions for the rest. It takes a great deal of trouble to equip an army: the US only had made about 800,000 of the Model 1903 Springfield rifles at the outset of the War, and had to contract with Remington and Winchester for the simpler and cheaper P-17 Enfield to supply the 2 million men it sent over. And the US was lucky: few anticipated how much materiél the war would demand, how industrialized it would be, and there was a scramble for arms. France and Italy would break out obsolete black powder rifles from storage and convert them to modern cartridges. And everyone was trying to produce as many artillery shells as possible. So there would have been no chance of switching over all the rifles in all the armies, even if the idea had been perfected, and embraced by the generals.

The submachine gun was created as a result of the War. It was realized that a rapid fire short-range weapon was often more useful than the very accurate long-range ones for use in the trenches ( the 1903 Springfield being a very good example of the latter). The Bergmann made its appearance at the very end: the Thompson just missed it. Unlike semiautomatic rifles, SMG's can be simple: the pressures are low ( using pistol cartridges) accuracy is not critical so they can fire from an open bolt, i.e., a heavy breechblock is forced forward by a spring when the trigger is pulled, taking cartridge off the magazine, loading and firing it, which spoils the aim. Don't know if they were less trouble than the Mondragon ( the later Sten guns didn't like dirt, either, and the Bergmanns had a bad tendency to fire if dropped or slammed from the rear). They were much cheaper to make than the Mondragon, and Germany was able to put about 5,000 in the hands of the German storm troopers, where they were effective.