Ranged PCs arent providing an important function they've always held. That of meat sheild.
Meat shields are easy to create in 5E. As mentioned previously, a range-specialized PC who puts away his bow and spends his action donning his shield becomes a meat shield. After that he can either draw his rapier and attack, or draw his rapier, Dodge, and threaten opportunity attacks. It depends on the terrain. Other options for meat shields include conjured animals, hirelings, armored zombies or skeletons, and elementals. A ranged party can generate meat shields when necessary, but a melee-heavy party cannot generate ranged firepower nearly so easily when it is needed.
Assuming the DM is going to place monsters (at least from time to time) in melee range, those monster attacks are going to be targetting something. You need a guy who can take those hits and tank it up, if for nothing else than to stop the monsters wailing on the mages.
This in bold is the core difference between our games. You run a hack-and-slash game where the DM plops down monsters in front of the PCs, makes Balors appear in the PCs' midst, etc. The DM's hand seems to feature very heavily in your scenarios, and apparently the players are 100% okay with it because they're really just looking to roll some dice and have lots of fights every day.
I run a combat-light game where combat, when it happens, is intended to be an emotionally significant event. I don't "place monsters in melee range"--the monsters either enter melee range somehow, or the PCs enter the monsters' melee range, but both the monsters and the PCs exist before conflict occurs and may still exist after it. (Conflict may or may not lead to combat, and combat may or may not lead to death.)
If the PCs have someone (a Mobile Alert Shadow Monk, a Rogue, a Druid, a Sprite familiar) on point scouting ahead, and are exercising reasonable care in the main party's movement, they'll know about most threats long before the main party enters melee range; and many of the threats they don't know about won't ever learn about them either. There may be cases where e.g. an Ankheg or Black Pudding or Intellect Devourer detects the Shadow Monk with Blindsense or Tremorsense or psychic senses, but in that case the Shadow Monk just has to break contact, which is fairly simple for someone with +18 to Stealth, +9 to initiative, immunity to surprise, and virtual immunity to opportunity attacks. Against normal foes like giants, demons, dragons, a party with a point man will always have some warning. My games are designed to reward realistic intelligent tactics and create an immersive fantasy world which is sometimes more dangerous than the PCs are: Combat As War.
My impression is that if someone tried that in your game it would turn out to be futile, and the party would often find itself ambushed at close range by monsters anyway. From what you've described previously about yourself I think this is because your players expect a Combat As Sport experience: lots of fights, all tilted in the PCs' favor, supplied by the DM.
