One player falling back 30ft while everyone else continues fighting is different from the group deciding "this looks bad & I think this is a fight we are going to lose, lets run away before this gets worse" . When the group chooses to run as a group they pull out all the stops going for broke to get somewhere safe so they can rest up far away, but just one player choosing to run to the other side of the room or something while the rest of the group continues to engage in a fight they expect to win is just fingers crossed tactical positioning. In some past editions characters were sticky because moving more than 5 feet/1 square provoked an opportunity attack & in some cases it provoked one for each 5 foot/square moved through up to dex mod number of them per round on top of monsters with less HP so that tactical positioning was meaningful but it certainly wasn't running away then & definitely isn't in 5e.I take umbrage with running being easy. Every time I see it attempted, without a very specific ability such as a caster dropping a spell that can slow down pursuit, the enemies just catch you. While there are chase rules in the DMG, the line between "when combat ends" and when it's a chase now, is unclear.
Every instance I've seen of players attempting to run has had to deal with the combat rules. If you just turn and move away, you provoke opportunity attacks. If you withdraw, the enemy just moves up and attacks you again. It's like the old adage, you don't have outrun the enemy, just the slowest party member, but that leaves you without party members.
Plus, there are monsters who are faster than your party members. Quite a few of them. In another thread, I postulated the issue of a monster like the werewolf. If you encounter one at low levels without silver or magic weapons, people said "oh you just run away and come back prepared". Except, in wolf form, the creature has a speed of 40. Who is running away again? The Barbarian or the Monk?
Now if your caster can drop something that slows enemies down, or there's some stunt you can pull to do so, that's one thing, but for a guy stuck in melee with a monster, he's going nowhere.
Resting up & recovering in 5e is so trivial that of course a group will have trouble running away when they try to tread the line between falling back to a closet somewhere to rest up when half the party is still doing their best imitation of a wackamole arcade game that's already been lost but is forcing the GM to run the trainwreck down to the last meeple rather than conceding to flee simply because they never committed to either. When the group plans & proactively commits to a strategy like "lets all run & hopefully it will chase Alice/Bob when we scatter because they can use $ability to probably get away" early the GM tends to be quite willing to help your strategy work for reasons colleville does a great job of explaining "Why?"
In the past there were crunchy bit of combat that combined with things like resource attrition & a higher chance of lethality pushing players to collaborate & plan during combat. That collaboration & planning made sure they could all be aiming for the same plan things like when to bravely run away instead of conflicting plans, but that's pretty much gone by the roadside in 5e in favor of just soloing near each other with no concern for life limb or resources. Players rarely ever consider actually running before the fight is already settled & the corpses are just waiting to fall. During the discussion about what went wrong & what could have been done players can talk about making sure that the group considers working together & thinking about those resource expenditure things more will help avoid a repeat by running earlier. By making it clear that the group's goal is to run like hell before the result is cemented it makes "the line between "when combat ends" and when it's a chase" crystal clear from orbit even to the most distracted GM. If the line is ever still unclear when a chase starts players haven't shifted from tactical repositioning to a run away commitment & can bring that up in the discussion among players after the dust settles.
Then there's the example. A werewolf is cr3 ac11 58 hp & makes two anemic attacks (1d8+2 & 2d4+2). Cantrips alone can handle a werewolf without much effort by a group unless the group is so low that things like a few zombies alone are a meaningful encounter. No group with more than a couple levels should have much trouble dealing with a werewolf unless the encounter is edging into LMoP's goblin ambush from stealth on 1st(?) level PCs type territory. CoS starts out hunting them with some start options but I can't see how anyone can even be injured by
Strahd is using the werewolves to lure adventurers to his domain. Characters can follow the werewolves' tracks into the Misty Forest. After hours of fruitless searching, the characters are engulfed by thick fog:
The woods darken as the trees begin to close ranks, their needle-covered arms interlocking to blot out the sun. The shroud of mist that covers the ground turns into creeping walls of gray fog that silently envelop you until you can't see more than a few feet in any direction. Soon, even the werewolf tracks disappear.
No matter which direction they go, the characters come to a lonely dirt road that cuts through the woods, leading to area A (see chapter 2). As an alternative, you can have them enter Barovia near Krezk (see chapter 8).