I don't know who you think you're disagreeing with here.Let's say there's a group using flashbacks in their 5e game...and, they're making it work for them. Are they playing it wrong? Is their game "incoherent"? Would they have more fun, objectively, if they didn't try to mix and match mechanics, principles, and influences in that way? I think my approach there would be to say, ok, if the group is having fun and making it work, let's assume they know what they're doing. Let's look at that as a legitimate playstyle, and then go from there, if we wish, to figure out what makes it fun.
But one might ask in what way is the group using flashbacks?
Here's the first post I know of on ENworld that canvassed a flashback-type mechanic in D&D, from about 13 years ago:
It triggered some sceptical responses, but not from me! Mostly from the "living sandbox" crew. (Some of whom objected to the suggested Acrobatics check too.)Yes. The skill checks have to be using appropriate skills. This is a roleplaying issue.When posed with a problem like the door to Moria would you use your Diplomacy to pass it? Would you use your Acrobatics? Would you use Arcana?
The simplest way to use the skills to meet the challenges you face is to use the correct one for the skill check.
But which of Diplomacy, Acrobatics or Arcana is the correct skill? You (the player) tell me (another player, or the GM).
Using Diplomacy: "Remember that time we were visiting the Wizards' Guild in Greyhawk? And I was buttering up that Burglomancer specialist? She told me a heap of old magical passwords - I try them all." The player rolls Diplomacy (probably at a hard DC - it's a pretty far-fetched story!) to see if this is true.
Using Acrobatics: "As the Watcher in the Water writhes about with its tentacles, I dodge at the last minute so it smashes into the door and breaks it." That might be a hard DC as well.
Using Arcana: "I speak a spell of opening". Medium DC. Or "I speak a spell of recall, to remember all the passwords and riddles I've learned over the years". That's more interesting and more clever- let's say a Medium DC with a +2 circumstance modifier.
My first thought about adapting this to 5e is that, in the absence of a skill challenge framework, what gives "teeth" to a failed attempt at a flashback? (In a skill challenge, obviously, those teeth arise from the fact that a failed check racks up the overall failure tally.) Deducting a hit die, or imposing a level of exhaustion, would seem a bit odd - why does regret, now, that one didn't pack the needed item back then cause one now to be sore and tired?
The closest thing I can think of to a generic playerside resource to be taxed on failure would be Inspiration. So maybe the rule would be to declare a skill check with a flashback component requires having a point of inspiration available; if the check fails, the inspiration is spent.