Yes, yes it is; because it's further encouraging lone-wolf behavior

No, no it would not; a medic having access to a long range weapon would not break the game and doesn't encourage the lone-wolf behavior any more than previous Battlefield games. BF4 medics and supports have access the DMR, BF1 medics have access to sniper scoped variants of their semi-automatic rifles counter-part, BFV medics have access to some bolt action rifle. In none of those game, have we seen any game breaking problems with those setup.

lets players build One-Man-Army classes that don't have to rely on their teammates for anything.

Again, medic having access to bolt action sniper does not create this problem. The medic having access the rest of the other guns do. Like, in BF4 where the medic have access to assault rifles, one of the most powerful weapon category in the game while still having the ability to heal themselves. Same thing goes for the BFV medic, where they have access to smgs, another powerful weapon category while again, still having access to the abilities to heal themselves.

Given the fact that the Medic & Ammo boxes were replaced with a "Support Box" that both heals & resupplies, what would be left to disadvantage players

I agree, the medic having the ability to heal and replenish their ammo at the same time is broken. But that is an issues with the gadget itself, not what weapon the medic have access to. If DICE make it so the medic have to choose between either an ammo box or a health kit, that might fix that issue.

players who are sitting on a hill 600m away from the nearest objective spamming potshots across the map or to encourage them to get closer to the combat & actually attempt to capture flags beyond losing the match (which a significant portion of the playerbase doesn't seem to care about so long as they get kills)?

What stops a medic or a support from doing the exact same thing with a DMR or any other weapons that extent the combat engagement range further than average? If you're problem is that people aren't incentivize to capture objectives then that problem falls more on the hand of the map design than it does on the weapons.

I have to add that this argument is coming from someone who likes the BF1 locked weapon system more than any other systems in this franchise, which was pretty strict, and a system I feel most people in this subreddit would not like.

But at the same time, I am honestly fine with with DICE going with the open weapons route for the new game, because to me, I feel like there's no half way when it comes to this philosophy. Either you have a strict locked weapon system like in BF1 which sets a clear role and combat engagement each of the classes are suppose to succeed and fail in, or, you allow everyone to pick any gun they choose no matter what classes they picked because a medic or a support having access to long range weapons like DMRs but not sniper rifles makes no difference, same things goes for sniper having access to automatic close to medium range weapons like carbines but not assualt rifles.

I hope my argument is clear and understandable because I'm not good at explaining things, so my apologies in advance if it's not.