The Halo 5 magnum does nothing that previous utility weapons haven't done before, there are just even more redundant clone weapons than ever before.
You can fix fundamental design flaws by just tweaking some weapon values, eventually the best variant is going to float to the top. Despite all weapons being generally lethal, basically none of them have anything to offer that can't be found in another near identical weapon. And no, being marginally better at marginally different ranges doesn't count.
Without radical changes to how weapons in the sandbox function that is never going to change now matter how many balance updates you have, if not the Magnum then it would be something else and spawning with a sub par weapon that is outclassed by a pickup weapon doing the exact same job isn't an option.
Weapon choice can't be a matter of pure upgrades or merely for the sake of superficial "VaRiEtY." It should be based on more than which interchangeable utility weapon or bullet hose is prettier.
TL;DR Whining about the Magnum is a shallow waste of time when the shallow clone filled sandbox has always been the real problem since Halo 2.
Well, to begin with, I'll repeat the same again; using older games as an argument to change an already functional and balanced gameplay doesn't make it truthful. The old concept was flawed in some parts; the reason might be the amount of "cloned" weapon as you said, paired with a too strong utility weapon start that made it pointless to pick up another weapon from the scenary unless it was a power one or you run out of ammo and had no chance. Usually dual wielding also failed to fill its purpose which made it unpopular and exclusively close range based unless you had strong coordination and knowledge of the best combos. Even energy guns had problems outside of their meant range, while your starting weapon was capable of fighting everything in all scenarios quite easily if you knew how to do it (with a bit of skill and luck, of course). So, as I see it the old sandbox balance isn't anything reliable in the first place.
It's true that if you tweak the sandbox, something is gonna float to the top. I see your point, and it's easy to see in the old H5 balance too; the BR was constantly complained about, and then developers decided to nerf nearly everything except the Magnum, so that the pistol now had that OP/really useful place. In my opinion the only thing that need to be changed slightly was the BR, the Magnum was fine in the old sandbox and there was no point on nerfing the AR range and damage capabilities (it even struggles up close due to the spread and inconsistences), the SMG for that matter (given that Shotgun completely outclasses it), the Storm Rifle (although it's somewhat fine right now) and the Carbine. I'd even argue that the BR got too much of a nerf; it's a weapon that's in on the map after all, so it needs to be slightly better than your current weapons OR offer something mechanically different while having similar power, as you said. Whether you think it's a valuable weapon upgrade or not if you take similar weapons but with slightly more power is entirely subjective, just like this thoughts. A "clone" can still be meant for some ranges and be good for that purpose and some gameplay tactics, so as long as you have a point to upgrade your current weapon it doesn't necessarily have to offer an extremely different mechanic, at least in my opinion and that's without mentioning that you're looking for a gun that performs the same task but with something different, not a radical change in gameplay. You just need one of the two conditions. This is why someone prefers a Carbine over a BR even in the most balanced and old scenaries.
Furthermore, variety is not the point here. Sure, nerfing the Magnum or going back to old weapons (before the update) with a recently tweaked BR can help with that, but variety eventually increases when health is the major feature of the sandbox. That's why the majority of the guns are still getting used despite being very similar in their roles, only the SMG, AR, Gunfighter Magnum and BR are getting underused and overlooked during most matches. The sandbox is still really accurate and even in most cases, so it just needs small tweaks such as the ones I mentioned before. Moreover, I've not seen any post in this thread that points directly to which gun is prettier, that's pretty pointless in my opinion as the main purpose of picking up another weapon is upgrading your weapon with one of the same role but with a bit more of damage/rate of fire/precision depending on your preferences. Obviously, a different or new mechanic is something to look at but it's not currently possible with this sandbox, and again you're looking for similar roles and functionality. And why is a good weapon still a bullet hose? Never saw a complaint like that in the older H5 meta.
To sum up, personally, I think going back to the old balance where the sandbox was pretty balanced is the best and easiest decision. As for clone weapons, let Halo Infinite fix that flaws, as I see it the guns can still hold their own right now without featuring radical changes. My point for a sandbox to work is just healthiness and usefulness, paired with a strong but not too much utility weapon + a reliable auto start (such as the old H5 AR, a deadlier version) and slightly buffed guns/mechanically different if possible in the scenary (you need a point to pick them up, control their spawn spots and to use strategies to take them). Currently, I think the only precision weapon that fits that conditions is the Light Rifle, given the amount of power the Pistol has right now. And having to fight against a team of 4 that has the whole upgraded weapons, given the worst and unusual situation, then trying to steal them needs plenty of tactics, and that means skillful matches. As soon as one of them dies, the balance returns, from another point of view.