What is reasonably balanced though?
Because even with terrain, varied encounters, and good use of player tactics, there have been plenty of rounds of combat where my greatsword wielding fighter had to dash just to engage an enemy. In our last game for example, out of ~16 rounds of combat, I spent 3 of them with no enemy within melee range. During these rounds, the ranged PCs were all still able to make attacks.
Sure. If you don't equip your melee fighter with any kind of ranged attack at all, then he won't be able to make ranged attacks and will instead have to do everything he can to close with his enemies. Maybe give him some javelins or handaxes to try and mitigate his lack of range. Sure, such attacks aren't going to be as effective as a ranged attacker's shot, but it's better than nothing.
There are a number of occasions where a melee fighter simply will not be able to contribute meaningfully in combat. This is undeniable fact.
There are a number of occasions where a ranged fighter simply will not be able to contribute meaningfully in combat. This is undeniable fact.
Both of these statements are true. The question is the frequency of each. It seems many tables experience more occasions of the melee fighter not being able to contribute. This is the DM's responsibility to recognize and, if it is an issue, to correct.
Some fights will start at more than 30 ft away. Sometimes enemies will spread out to surround the party or avoid AoE. Sometimes there will be squads of enemy ranged troops who prefer to fight at a distance. Sometimes there will be flying enemies that face the party. Sometimes enemies try to flee and need to be stopped. The ranged fighter suffers no loss in efficiency in these scenarios.
Sometimes enemies drop prone causing disadvantage to attacks from ranged fighters. Sometimes, enemies will step behind a rock and gain full cover, rendering ranged attacks useless. Sometimes enemies pop out of cover, fire, and then back to cover, forcing ranged fighters to ready actinos in order to get one shot at them, which limits the number of attacks. Sometimes enemies will be able to move right around the ranged combatant because they don't threaten with an opportunity attack.
Again, the DM needs to do what he can to create encounters that give all characters a chance to shine. If the entire party is made up of ranged fighters, then all fights can focus on ranged. If everyone in the party is a melee fighter, then ranged concerns aren't as big a deal. But if there is a mix....then the DM needs to mix things up.
IMHO the best solution to making weapons more equal in terms of power is this:
a) change the -5/+10 part of GWM and sharpshooter to impose disadvantage on your attack for + 1W damage (this ability can't be used if you already have disadvantage on your attack).
b) You provoke opportunity attacks when you cast a spell or make a ranged attack. Also, don't allow feats or fighting styles that prevent this OA.
These seem somewhat reasonable. I think the second is easier to remember and enforce....ranged attacks provoke opportunity attacks from adjacent foes.
A few other things to note.
The crossbow archer can can carry a number of mundane daggers on his person. As such he can draw a dagger at the end of each turn to be used for opportunity attacks. A d4+5 damage opportunity attack isn't great, but it's still about 70% as effective as a d12+5 damage OA.
Combine this with the fact that OAs only occur very infrequently in actual gameplay and the whole argument about archers being unable to "tank" as well as a melee fighter becomes plainly visible as the yet another post truth perpetrated by those who have no grasp on how 5e combat actually plays out.
So the loss of efficiency for a ranged fighter to make an OA with a dagger isn't a big deal, but the loss of efficiency a melee fighter takes for making a ranged attack is terrible?
There is no need to say that folks who disagree with you have no grasp of how combat plays out in 5E. I mean, my combats play out where there are many opportunity attacks made, and the threat of them is almost always a factor. That's how combat plays out at my table.
That doesn't mean that I get to accuse you of not grasping how 5E combat plays out. The answer is it plays out differently at different tables. Your table has an issue with balance between ranged combat and melee combat.....no need to accuse others who don't experience that problem as not knowing what they're doing.
I mean....it seems odd to tell someone who isn't having a problem that they're the one doing something wrong, no?