Rouroni, appreciate the fun DEEP DIVE on this subject! 
You mention that the mechanics for the Defense Reaction against a ranged attack using Acrobatics isn’t the same as using Parry. Obviously Acrobatics and Parry have their own talents, but mechanically the structure is the same… I’m open to why specifically it’s mechanically different, but I can’t find anything. Also, I don’t think using Parry necessitates “to bat it aside” as there’s nothing in the rules to prevent an unarmed target from using Parry against a melee attack (note that unarmed strikes are given a Reach of 1 on page 152). Parrying would reasonably include stepping clear of a melee weapon swing.
On that note, I agree with your point that it’s a bad idea for a GM to allow Acrobatics to be allowed for dodging a melee swing. Parry works fine for an unarmed combatant (or anyone) “dodging/stepping aside” from melee attacks.
You mention that “the way the system is set up ranged attacks have a significant advantage”. Obviously ranged attacks have the intrinsic advantage that they are effective at range - but I’m pretty sure that’s not what you’re referring to when you say, “the way the system is set up.” So I’m not sure what “significant advantage” ranged weapons have over melee weapons you’re referring to? Are you referring to the Volley Quality? That doesn’t apply to all ranged weapons…I don’t think I’d consider that as constituting a “significant advantage” over melee weapons.
As I’ve mentioned, completely agree that a ranged-weapons fighter should suffer a disadvantage against a melee foe while in Reach - and the rules stipulate this as always being true by always raising the Difficulty of the ranged attacker when targeting any foe in this situation. Therefore our question here, is not whether the ranged attacker should suffer disadvantage in melee, but how much.
You mention that one of biggest functions of Parry is the ability to retaliate as well as other possibilities
that it opens up with other talents. There are no Parry talents that involve the Retaliate Reaction. Two of the other talents involve Stage Fighting, so not applicable. One involves using Parry as a Standard Action, so that’s non-applicable for the Reaction situation we’re discussing. One talent lets you use Parry instead of Acrobatics against ranged attacks, which also is not applicable to our situation here of a ranged attack in Reach of a melee threat. One talent makes the Defend Reaction cheaper - I suppose that indirectly supports a possible later retaliation of some kind? There is one Parry talent, among the six, that does facilitate an immediate counterstrike (Riposte). That’s a good talent, but I can’t say I’m onboard that one of Parry skill’s “biggest functions” is essentially the use of this one talent. Let’s keep in mind that Acrobatics offers its own talent advantages in combat, including allowing Acrobatics being used instead of Parry during Defense Reactions against melee attacks, as well as offering a 1d20 reroll talent. (Dutifully noting however, that it does not have that one Riposte counterstrike talent that Parry has.)
I hear you on your just go case-by-case point, nothing wrong with that. I’m not interested in having a rule to cover every possible eventuality no matter how rare. However, in my view, a ranged attacker being in Reach of a melee threat will be common enough to warrant figuring out in advance how best to handle it in most cases.
However, at the end of your reasoning you return to that “it makes sense to assume that ranged attacks trigger a retaliation.” You’re still again saying that while Modiphius actually used “may”, that we should trust you that they actually really meant “pretty much always”.
In my opinion, that’s an unreasonable leap. I do not believe the rules assume that ranged attacks trigger a retaliation (or else they’d say that), and nor do I believe the rules assume you’ll do it as I would (see below).
I take RAW on this as straightforward as I can. The rules put in a floor on this situation by stipulating that a ranged attacker will always increase the Difficulty of their attacks against all targets as long as they’re in Reach of a melee foe. After that, the “may” in question is saying it’s up to the GM if and when Retaliate Reactions come into play.
Lastly, to that end, note that I’m not taking the opposite position from yours, I’m just not going as far. I completely agree that a ranged attacker should be at disadvantage when a melee threat is in Range - and the rules stipulate this is always the case, with 1 increased step of Difficulty for the ranged attacker when they’re targeting any foe while a melee threat is in Reach. Regarding Reactions, I would have the Retaliate Reaction available to the melee foe when 1) the ranged attacker targets someone besides the melee threat in Reach (thereby is focused on someone else) and/or 2) uses the Exploit Action on any target, including the melee threat in Range (thereby in essence holding still and aiming).
Thanks again for the discussion Rouroni! Good stuff. Cheers