It's a conceit necessary to make 5e Combat work.
PHB p 189: A typical combat encounter is a clash between two sides, a flurry of weapon swings, feints, parries, footwork, and spellcasting. The game organizes the chaos of combat into a cycle of rounds and turns. A round represents about 6 seconds in the game world. During a round, each participant in a battle takes a turn. The order of turns is determined at the beginning of a combat encounter, when everyone rolls initiative. Once everyone has taken a turn, the fight continues to the next round if neither side has defeated the other.
Could you explain how simultaneous combat actions work in your game? It has been decades since I played 1e.
Do players declare what they are doing up front, DM declares what the NPCs/monsters are doing, and then the DM resolves everything at once?
Our initiative system (homebrew) works fairly simply. Everyone rolls an independent d6 at the start of the round for each action they have in that round (thus e.g. a warrior using two weapons would roll a separate d6 for each one to show when those attacks occur). You usually don't have to declare what you're doing until your init comes up unless it involves significant movement.
Then, I-as-DM go round the table and ask if there's any sixes. Two sixes, OK. Both those players take care of whatever they're doing on a six, and we sort it. Next, I-as-DM see if the opponents have any sixes, oh look, I have a six - and even though one of the players' six-init actions finished off that foe it still gets to act on its six because it's simultaneous in the fiction. I take care of resolving that six, then we move on to fives, repeat the process, and so on down to ones. For the next round, all initiatives are rolled again.
If you're casting a spell you start on your rolled initiative then the spell's casting time (always listed in segments, of which there's 6 per round corresponding to the pips on a d6) determines when you resolve. Thus, if you roll a 5 and start a 3-segment spell you resolve on a 2; if instead you rolled a 1 for init your spell won't resolve until 4 of next round.
If two spells are resolving on the same segment they both resolve, unless one caster is targeting the other. In such cases I often do need to determine which caster was that split-second faster as the other caster's spell might be interrupted just as it's about to resolve.
And my game does have counterspell as a spell, one of the very few with a 0-segment casting time. But, there's also a hard rule that says another 0-segment spell cannot be countered as it is too fast (in other words, FIFO resolution), meaning you can't counter someone else's
Counterspell,
Command,
Featherfall, or other 0-segment spell.
It's a bit cumbersome if you're new to it but goes pretty fast once you know the ropes.
Incidentally, spells like shield and counterspell do allow for "two or more things happening at the same time in the fiction", as you put it. Yet you don't seem to like those. Why?
I don't mind those at all in principle. What I very much dislike is how 5e allows them to resolve on a LIFO basis.
