If there's only one enemy monster "theater of the mind" may work out.
You can see this, when actually doing a "one enemy monster fight" on a map. It's often very boring. The fighter/paladin/barbarian tanks all the damage, everyone goes to their position and never moves again. yawn

If there is more the one enemy monster, ranges are of importance. That is what a map is for.
The rules don't say:

You can cast this spell on everyone in [arms reach/initimate range/seeing the white of their eyes-range/view-sight in general].
It affects the target, and everyone in arms reach around them.

The rules say:

You can cast this spell on everyone in a 50ft range – it affects the target and everyone in a 10ft radius.

If you're not using a map when ranges are this specific you are bound to break rules, because you inadvertently will get a placement wrong. Especially when AoEs with ranges are involved it often becomes a huge clusterf*ck of "Ooof, who does this even hit? Who's in a 10ft radius?".
So it often ends in GMs being nice to their players and throwing the rules overboard, saying the spell hits everyone they want.

If DnD tells you: You can move 30ft, and the spell has 50ft range, but you're winging it, you're playing the wrong game.
Do it rarely: Okay, no need to change systems.
Do it all the time: Change to a system with less of a strategic element. You obviously don't care about movement-ranges.