The thing is, you have to realize this is a "MMO" with its playerbase mostly absolutely despise the idea of MMO and what it stands for.
So a global chat will likely cause many of these kind of players to rage and complain about how it breaks their immersion. Considering people rose pitchforks against CQC rank being in the main game and wanting to remove it completely just because it breaks their immersion, I think something like this will raise hell over the forum.
You're correct, but not for the reasons you claim.
It's less about immersion, and more about that universal chat, like CQC being explicitly tied into the main game, breaks one of the central defining properties of the Elite Universe: most information in the galaxy travels aboard ships.
This property is why people are willing to pay you hundreds of credits to deliver information for them. Intersteller communication isn't even at the level of the telegraph, let alone the internet. If you want to send someone a bunch of pamphlets in a system five light years away, you can't simply email them to a local printer and have your friend pick them up. You have to get someone to hand deliver it.
This property is also why the mega-corporations that rule over most of humanity are able to get away with their monopolistic practices and their policy of crippling over-specialization. Their exploitation of their workers/citizens would never succeed in a galaxy where the free exchange of information was ubiquitous.
This property is also why the Pilot's Federation is even able to exist, let alone be the 4th major superpower in human space, able to change the course of wars and alter the fates of billions of people. This property is why the Pilot's Federation is able to operate outside the law of any government it has a treaty with.
I generally have no problem explaining the more game-like compromises required to allow Elite: Dangerous to operate as a (massively) multi-player online game.
The Frame-Shift drive is clearly new technology, largely confined to the Pilot's Federation and major powers in the game. Most of the galaxy is still using the slower hyperdrive technology from Frontier First Encounters, and those USS's you sometimes see while zipping along in Supercruise are those poor unfortunates who are still taking two or three weeks to get to another system.
The local instanced chat is a side effect of the FSD, allowing radio signals to propogate through Witchspace locally, which can be picked up by people nearby who are also in Witchspace (using a FSD), but not people in normal space.
Direct Chat and Galnet is even newer technology, a monopoly of the Pilot's Federation, and largely the high-tech equivalent of a mirror on a hill flashing morse-code at the next hill top.
If information were to flow fully to a degree that galaxy-wide chat was possible, then my sense of immersion wouldn't be the victim. The victim would be my suspension of disbelief that the aspects of Elite that make it stand apart from the "Generic Space Sim" are still around. The free flow of information would destroy the mega-corps' exploitation of their workers, and the leveling of commodity prices would turn systems into generalists, because it would be cheaper to build the infrastructure in-system than transport it from other systems. The Pilot's Federation would lose its status as the largest legal criminal organization in the galaxy, and its members would lose the protections that status brings, and would be reduced to a freight hauling company for the few items that can't be produced locally and luxury items.
