It's mainly a problem because it'll change around the type chart, which is obviously a change people do not want.
You may argue, yes, that the Physical/Special split was a huge change, and also that natures and even revising the DVs/IVs system were all big changes, but here's my main point: Since Gen. 2, the game has went without this Light/Good/Holy/Whatever type people thought was going to be a counterpart to Dark/Evil/Aku.
to be fair, I was not advocating the Light type (I did state I found it to make the most sense should they add one, but only because it's the most obvious element that isn't already represented), but rather a new type being added in general, regardless of what it was.
By that basis, I find it quite strange that Light's the one that sticks out like a sore thumb to things like "Wind" and "Sound" being speculated. Why Light of all concepts is the one getting the most speculation and opinions, is it just because of Dark, or is it that we just don't have a lot of "heroism" being speculated.
I think it's because Light is a common element used in pretty much every other RPG. it's just such a prominently used and obvious element, that the fact that it isn't a type yet is kind of silly. every other type that could be there is either really obscure, is too exclusive to make into effective typing (like "Sound"), or already there to begin with. there are other types that would work (Wind could work, although flying pretty much has that covered in terms of concept), but Light is one that just works so well that there's really no need to look any further. Light being the opposite of dark doesn't matter much if you ask me, because Light holds it's own as a typing.
NOW, onto some major points!
Now, my main point revolves around its inclusion, which you said isn't too big a problem. What I believe would happen is two things: A) Multiple pokémon's typings will change (Unlike from Gen. 1 --> Gen. 2, Magnemite's line was the only one with a change. Think if things like Cresselia, Lucario, Togekiss, and many others had to get their typings change. It isn't like the Physical/Special split where moves simply work on different attacks, it's Pokémon having their typings changed by an inclusion of one type. And it wouldn't be a small pool, trust me, it'll still be bigger than the percentage of Magnemite's line/251%),
Granted, this will cause major changes, and could cripple or buff Pokemon depending on how it changed. that being said, Pokemon will be crippled/buffed by new moves/Pokemon anyways. depending on if they make major changes to the core mechanics or not, that may effect them too. a new type may effect them more directly, but overall it's impact is only slightly more then what we already expect. people would adjust to it, and move on. there's also no need to change the typing of existing Pokemon, although I admit it would be very likely.
B) Moves' types will have to change (Which, tbh, wouldn't be too bad, though the moves that'll be changed to Light will be few, and said moves off the top of my head aren't even that popular, save Aura Sphere),
agreed, it's not that much of a change. most would be new moves, like Dark and Steel moves were.
and C) The type chart (As people argued, it sounds decent on paper, though there's no way to test it).
I really don't see how this factors into it, as it would obviously be tested and changed slightly to assure the new type works well with other types. it would either be good or bad, like every other type.
Now, my main gripe is still, throughout all these generations, why now do we still argue about Light? In the beginning, there was (And this is just a certain sector of the fandom telling me this, not sure if this applies to all) the thing that Espeon was a good, a "Light" pokémon because of it being blessed by the Sun, while Umbreon was tainted by the darkness and the Moon, making it an evil Dark type. There was discussion in 3rd gen, where Jirachi was "supposedly" heralded as almost being a Light type. And even recently in the old Gen. 4, where, from the amount of legendaries, many people guessed we were going to get a Light type, but we did not.
I must say, that this argument really doesn't make any sense. basically, the argument is that if they were going to add another type, they would have done so by now. in other words, "they shouldn't do it, because they haven't done it yet". hopefully I don't have to explain how silly that sounds. I know this is directed specifically towards people expecting a Light type, but what I said still applies; just because we don't have something doesn't mean we can't, which is probably why people still speculate a Light Type.
Out of all this, I'm personally miffed by the amount of support the Light type gets, as if it were some weird mantra that simply can't die down. Yes, I do not wish for it to happen, but from the arguments for it, I still find it a pretty ridiculous ideal, and if it does or does not happen, I'll remain neutral to its inclusion/exclusion from the series. It's just a strange topic that, whenever it comes up, continues to rile me up.
Long story short, I do agree with your arguments and what you've brought up, but at the same time, I'll still lean to the Light type not being included, but find it still possible to happen.