My worries about how easy this would be turn mostly on issues of resource-tracking: the more that resolving spell durations, healing, torches, iron rations etc takes you out of the crunch of the encounter, and into the more exploratory aspects of the game, the more you've lost your focus on encounters/scene-framing.
And also, drawing on what I said just above, all previous effects end (with a few exceptions, eg Disguise Self - but very few exceptions that interact with the core action resolution mechanics).
If what you say can be done, and the mechanical effectiveness all still make sense, then that would deal with some of my worries. You'd also have to solve the resource timing/management issue (eg at the moment Next seems to use a range of durations for its spells).
I don't see durations as much of an issue. Like in 4e, each round takes up 6 seconds. Short rests take 10 minutes. 4e and 5e both lack the 10-minute Turn of B/X, leaving the passage of non-encounter time up to the DM. (This is pretty easy to hack, though. Encounter speed of 30'/round x 3 per B/X and you get 90' of exploratory movement if you want to institute 10 minute turns.) The upshot is, in B/X rations, torches and the like run on the non-encounter time frame of the turn. Once you get into an encounter, those things are no longer tracked while the encounter lasts. If tracking those resources is something you want to do, that takes place outside the encounter. If it's something you
don't want to do, you focus on the encounter (the only setting at which 5e has hard-coded rules for time and movement), and elide the non-encounter stuff that isn't relevant to your game.
Similarly, in B/X you have spells have durations in rounds, and spells that have durations in turns (or more). Any spell that has a duration in rounds is perforce an Encounter-paced power, because even a 20th level Wizard with a 1 round/level duration to his spell only gets 200 seconds - a smidge over 2 minutes. Thus, his spell is over by the next turn. There are many spells like this in 5e. Going through the list:
Aid: This is one with a long duration - 8 hours.
Anti-Magic Field: The maximum this can last is 1 hour, but actually it's on Encounter pacing because it requires concentration to sustain it. Thus in most cases it will last until the end of an encounter (short rest), unless for story reasons that encounter takes up an hour of game time.
Blade Barrier: Until concentration is broken, or 10 minutes. Encounter duration.
Bless: Until concentration is broken or 1 minute. Encounter duration.
Charm Person: 1 hour or until charmee is attacked by charmer. This seems kinda half-way. Not purely Encounter duration, but not Daily in duration like in previous editions.
Cloudkill: Until concentration is broken, or 10 minutes. Encounter duration.
Comprehend Languages: 1 hour. Between Encounter & Daily.
Darkness: Until concentration is broken, or 10 minutes. Encounter duration.
Daylight: 1 hour. Between Encounter & Daily.
Death Ward: 8 hours. Daily Duration
Detect Magic: 1 minute. Encounter duration.
Detect Undead: Encounter duration.
Dimension Door: I think this is an editing mistake. It says "briefly creates", but doesn't state an exact duration.
Disguise Self: 1 hour. Between Encounter & Daily.
Divine Favor: Until concentration is broken, or 1 minute. Encounter duration.
Divine Power: Until concentration is broken or 1 minute. Encounter duration.
Dominate Person: 1 hour. Between Encounter & Daily.
Flesh to Stone: Until concentration is broken, or 1 minute. Encounter duration (unless you go the whole minute, in which case, the target dies).
Fly: 1 hour. Between Encounter & Daily.
Gate: Until concentration is broken, or 1 minute. Encounter duration.
Guidance: 1 minute. Encounter duration.
Haste: Until concentration is broken, or 1 minute. Encounter duration.
Hold Person: Until concentration is broken, or 1 minute. Encounter duration.
Hold Monster: Until concentration is broken, or 1 minute. Encounter duration.
Holy Aura: Until concentration is broken, or 1 minute. Encounter duration.
Invisible: 1 hour, or until target attacks or casts a spell. Between Encounter and Daily.
Invisibility, Mass: Until concentration is broken, or 10 minutes. Encounter duration.
Levitate: Until concentration is broken, or 10 minutes. Encounter duration.
Light: 1 hour. Between Encounter and Daily.
Mage Armor: 1 hour. Between Encounter and Daily.
Minor Illusion: Until concentration is broken, or 1 minute. Encounter duration.
Mirror Image: Until concentration is broken, or 1 minute. Encounter duration.
Move Earth: Until concentration is broken, or 2 hours. Encounter duration.
Otto's Irresistible Dance: Until concentration is broken, or 1 minute. Encounter duration.
Passwall: Until concentration is broken, or 1 hour. Encounter duration.
Phantasmal Force: Until concentration is broken, or 10 minutes. Encounter duration.
Planar Ally: Variable, but increasing expensive - 100 gp/minute, 1,000 gp/hour, 10,000 gp/day
Polymorph: 1 minute. Encounter duration.
Prayer: Until concentration is broken, or 1 minute. Encounter duration.
Prestidigitation: Generally 1 hour. Between Encounter and Daily.
Protection from Evil: 1 minute. Encounter duration.
Rope Trick: 1 hour. Between Encounter and Daily.
Sanctuary: 1 minute. Encounter duration.
Shield: 10 minutes. Encounter duration.
Shield of Faith: Until concentration is broken, or 1 minute. Encounter duration.
Silence: Until concentration is broken, or 10 minutes. Encounter duration.
Sleep: 1 minute. Encounter duration.
Speak With Dead: 10 minutes. Encounter duration.
Spider Climb: Until concentration is broken, or 10 minutes. Encounter duration.
Spiritual Weapon: 1 minute. Encounter duration.
Stinking Cloud: Until concentration is broken, or 10 minutes. Encounter duration.
Stoneskin: 1 hour. Between Encounter and Daily.
Suggestion, Mass: Until concentration is broken, or 1 minute. Encounter duration.
Sunbeam: Until concentration is broken, or 1 minute. Encounter duration.
Telekinesis: Until concentration is broken, or 10 minutes. Encounter duration.
Time Stop: Until concentration is broken or 10 rounds. Encounter duration.
True Seeing: 1 hour. Between Encounter and Daily.
Wall of Fire: Until concentration is broken, or 1 minute. Encounter duration.
Water Breathing: 24 hours. Daily duration.
Web: Until concentration is broken, or 10 minutes. Encounter duration.
Zone of Truth: 1 minute. Encounter duration.
All other spells' durations are instantaneous, or until the end of the caster's next turn. Those are perforce encounter based. Of the remaining 61 spells, 44 effectively only last for an encounter (pure encounter duration). Another 13 a pseudo-encounter durations. Conceivably their duration would last through multiple short rests, but the party would really be running a gauntlet of encounters. To me, these seem a nice "hinge" duration. If you're focusing on Encounter scene framing and skipping through downtime, these are effectively encounter duration. If, however, you're doing more of a dungeoncrawl exploration type thing, they might last through two or three encounters. The time is short enough that it doesn't have to be tracked, unless your game tracks every minute. Then you have 3 Daily duration, 1 variable, and 1 unclear.
I'm not sure this is functionally different from 4e, in terms of durations.