My view point is a bit more complex.
It is as you say for the most part. In theory, a single lower can address several roles through a group of uppers.
It is, however, not always convenient nor practical.
If one finds value in it for defensive purposes, a more or less classical configuration should be considered and maintained in that configuration throughout it's career.
Nobody has time to swap parts around in a firefight.
Mine has morphed a bit since the 16" mid length that originally filled it was supplanted by a 10.5" pistol variant. These are fairly well dedicated due to their roles being considered critical.
Cheap ammo? ..... you want to know whats cheaper than 5.56? ... why that's 9mm.
originally, I was going to address this from a lego gun approach. However, with the tuning required for function, a conversion to 9mm might be best considered as another dedicated build. Once you have it running, and a load worked out for about 100 yard accuracy, you have an interesting range mistress on your hands.
the 9mm's performance seems to overlap the small game utility of 22 lr and tactical roles which justifies its existence. It's not just two pins, a buffer and a mag, so it conflicts with the lego gun ecosystem enough to demand a dedicated lower with the Colt SMG pattern conversion. Glock mag lowers exists, and are arguably a more practical approach due to prolific magazine availability. But then you have a lower that will absolutely not convert to anything else. It's still another good little instrument for performing the concerto suite for thunderpipe in the key of 9mm minor.
It'll take the 10 or 12 round mag of the Glock 26 for the sportsmans second movement, "waskally wabbits", building through the glock 19 and 17 capacities on up to the epic blastathon climax of the 60 round drum solo.
After the dedicated lower for various reasons, I come to the open platform lego gun in 350 Legend.
It too is technically dedicated to 350 as I ran out of interest for the time being.
I may do something 6.5 grendel in the future. It seems a fine choice to enhance the platform's performance. But for now, all other core needs are met via lever and bolt actions in various flavors from 357 magnum through the only government a man can trust ... 45-70.
