I'm curious to extrapolate an evolutionary reasoning. So far we have:
• Thousands of years ago: T-shaped visors were the norm, for whatever reason, on a large scale. The Mandalore's mask had one 4,000 years ago. Neo-Crusaders over the next few decades had them, too. The other bookend on this era, so far, is the Old Republic MMO, where we see Republic troopers with T-visors, and generic "bounty hunter" armor with one, too, that looks like a definite antecedant to movie-era Mandalorian armor.
• ~25 years ago: The clone army was, according to Lama Su, "for the Republic". The helmet shape, at the time of the movie's release, was speculated to be derived from Jango's Mandalorian helmet, but what if it instead was a newer and more refined version of the Republic troops' gear from when the Republic still had a standing military? And that both their appearance and Jango's Mandalorian design had their roots in what we saw in TOR?
• The exigencies of war demonstrated some sort of shortcomings in the Phase I design and within a couple years, bulkier helmets had shown up with expanded peripheral sensors and some sort of enlarged/increased air-filtration/internal air supply equipment (Phase II). There were also a good half-dozen or more distinct helmet variants for the evolving need for more specialized kit for varying combat environments.
• Over the next couple decades, the design was refined and expanded further in a streamlining attempt to reduce the number of unique pieces of equipment, each of which required its own parts store to maintain. By the time of the Battle of Yavin, pilots and armored-vehicle operators within the Stormtrooper Corps all had the same faceplate as their infantry brothers, for ease of maintenance and simplification of parts inventories. Light Infantry, Commandos, and Force Recon also had their helmets merged into a more-narrowly-related family. The most distinctive outlier was the "Extreme Environment, Low Temperature" helmet, which still had a lot of similarities to the helmet in use by the Imperial Starfleet's ground forces (and probably a derivative of the AT-DP helmet in Rebels).
• Somewhere over the next couple decades, the solutions to the problems with the Phase I helmets had been streamlined enough to be less obtrusive. Visual and nonvisual sensors were better integrated, air-filtration systems had been improved enough that less on-board Oxygen was needed, and what was carried on-board didn't need to be supplemented by external feed (improved rebreathers?), so the chin connectors were eliminated, and the troopers are back to looking like updated versions of the old Republic troopers of the Great Galactic War, or the more streamlined Phase I troopers of the Grand Army of the Republic who later became the first Imperial Stormtroopers.
Those of you familiar with Warhammer 40,000 probably know of the similar armor evolution from the Mark II "Crusade" suit to the Mark IV "Imperial Maximus" suit to the slapped together Mark V "Heresy" suit to the more advanced and largely-parts-interchagable Marks VI "Corvus, VII "Aquila", and VIII "Errant". It had evolved over a couple centuries to a very refined design, but then civil war exposed flaws in the design, mainly its complexity and refinement meant parts were scarce, repairs were time-consuming or impossible, and stopgaps were needed. The Mark V used a lot of the heavy armor developed for vessel-boarding and very simple systems. Later, as manufacturing was able to incorporate lessons learned and catch up with production again, the new variants were introduced nearly simultaneously, and were more "on the fly" minor fixes, or optional extras, or "hey, let's see how this works in the field", and years later most troops wear armor that is a mix of all three to one degree or another, depending on a particular Chapter's character or freedom for troops to individualize/optimize, or how far they are from resupply and the need to cannibalize to keep things running.
Even today, the military often uses what we would consider "obsolete" tech because the bigger components are easier to see, isolate, and swap out when in the field. So even if technology has only enjoyed the odd tweak or refinement here and there over the last several millennia in the Star Wars galaxy, what gets used where when can be very fluid. There can be a lag of years or decades, depending on many factors, between when a need is filled with what is currently easily available and when engineers have been able to refine/amplify/miniaturize that widget to be more convenient and less obtrusive. I'm willing to bet that, while the commonality of the Stortmrooper faceplate made stocking spare parts easier, fixing or replacing individual components inside it was probably very fiddly -- and that this new Stormtrooper helmet has much more "all-in-one" components replacing whole subassemblies that can be swapped out quite easily by the troopers themselves.
--Jonah
