I think E. coli K12 was more fortuitous than limiting, because it happened to have the right mutations to make genetic studies and biochemical studies very easy. It grows extraordinarily fast in the lab, and requires only a limited amount of sterility to prevent contamination. It can also be grown in liquid or on agar “plates”, which means that they can be grown in both huge amounts as well as in colonies formed from each individual bacterium, making quantitative study a lot easier. And E. coli has plenty of bacteriophages, that allow for the ease of manipulating DNA back before recombinant DNA technology was mature. Finally, E. coli also has interesting chemotaxis and motility, which made it a great model organism for studies of flagellar movement, sensory systems, and signaling.

I think ultimately these discoveries would have been made in other bacteria, perhaps, but some of them (such as caulobactor) certainly grow much slower (which would make screening millions of colonies difficult) and have a much more complicated life cycle than E. coli, which is about as uniform and elemental as one can get in bacteria without being too boring and limiting. Even though other biologists were working in other model organisms, the fact that a huge portion of the molecular biology was worked out much faster in E. coli is very telling about its power.