Ashrym,
Thanks for the discussion. You don't have to respond to the below unless you want to, but for the sake of argument I wanted to show some worked examples of ranged tactics.
A 20 ft room doesn't require dashes, for example, and smart opponents can box ranged character in without him boxing himself in.
Stipulated that ranged attacks don't work well when you're locked inside a 20' room. Everything I write below assumes that you have a secure line of retreat.
The problem with dashing is it takes your action to do it so it isn't an escape system that allows damage and the mobs closing in can close the distance faster than they can be killed OR move and ranged attack back on their actions. This has nothing to do with it being a deadly encounter because monster hit points in higher CR's have some pretty hefty scaling. A group of 5th level adventurers facing 2 ogres would take eldritch blasts for 4 rounds to drop one of them and if one those ogres attacks the warlock he'll get there before dropping.
The thing is that if you are a ranged party, you can plan not to be very vulnerable to enemies Dashing to close with you. You want to do this because not only does keeping the range above 200' make you less vulnerable to melee attacks, it also makes you less vulnerable to Medusa gazes, poisoned drow crossbows, Fireballs, Counterspells, and breath weapons.
Assuming an initial range of 100' (favorable to the ogres), and a not-very-optimized party of one Spell Sniper warlock, a wizard, a Mobile monk, and a paladin:
Prep: Wizard casts Longstrider on everybody (2 2nd level spells).
Procedure: Every round, the ogres close to 20' by Dashing, adventurers open it back up to 100' by Dashing. Wizard and Warlock both Expeditious Retreat so they can Eldritch Blast/Fire Bolt every round in addition to Dashing, inflicting 26.6 points of damage per round between the two of them (ogres have 59 HP). The monk could exploit standard cyclic initiative to get free melee attacks every round (that's one reason I prefer Speed Factor initiative where the exploit doesn't work reliably), but instead he'll Dash every third round and fire the other 2/3 of the time, inflicting 10.2 damage per round on average. The paladin doesn't do anything interesting in this combat and can be ignored. (He's insurance for the times when holding the range open
doesn't work.) Total party damage: 36.8 per round.
Time to kill ogres: 24 seconds (four rounds). Distance consumed: 320', just under a Seattle city block length (364 feet).
When I mentioned deadly difficulties, I wasn't referring to chumps like ogres but rather to cases like "6d6 giant worker ants and 3d6 soldier ants and 1d6 bomber ants, every ten minutes" (total of 1291 angry HP of ant on average) from Quests of Doom II. The party will do less damage per turn against the ants due to better AC, so call it 30 DPR, which means that it takes 43 rounds and 3400 feet of ant tunnel to kill the whole ant group--that's starting to push the bounds of believability that there would be so much tunnel available behind you.
If the ogres chuck javelins instead they will do it from long range (60') and only get to do it twice; after that the party is at 180' which is out of javelin range.
1/2 CR orcs with the aggressive trait eat up range like candy, and both ogres and orcs are examples of monsters that can simply return fire. Enemy spell casters, creatures with ranged attacks, creatures with fast movements, stealth attacks; or simply going with the dash and being within melee range so that eldritch blast attacks with disadvantage (or the warlock moves away and loses an action to dash / disengage while taking an opportunity attack) or forces a feat early as well.
Orcs aren't that much better than the ogres in this case: they close 90' per round instead of 80', which basically means you can kill 2 free orcs per 10' of distance over 90' that you start the combat at. (100' initial distance = 2 free orc kills, 200' initial distance = 22 free orc kills, etc.) Every 2 kills will also consume 80' of retreat distance.
I absolutely agree that enemy ranged attacks and spell casters, creatures with fast movement like dragons and rocs, and stealth attacks are all the bane of ranged characters. They're the bane of melee characters too but at least the melee guys are planning and expecting to get attacked, so they tend to have better AC.
However, I didn't follow what you meant by "the warlock moves away and loses an action to dash / disengage while taking an opportunity attack". If the warlock disengages, why is he taking an opportunity attack? Conversely, if he simply moves away and eats an opportunity attack, why is he losing an action? He should get his normal attack at no disadvantage.
Anyway, thanks for the discussion. It was fun.