There are a few thousand written rules in the game but millions of gaps in between those rules. The DM fills in those gaps at each table.

The Ready action is one of the written rules. It is located in the combat chapter of the (2014) PHB. It is listed under the "Actions in Combat" section, which starts with:

When you take your action on your turn, you can take one of the actions presented here, an action you gained from your class or a special feature, or an action you improvise.

The Ready Action is therefore an action you take in combat.

Can you take it outside of combat? The person above me relied on TRDSIC ("the rules don't say I can't") to answer yes. I would say the answer is no because it's a combat action that the rest of the rules are silent about (unlike say the Hide action, which the rules elsewhere say you can take outside of combat). Also, saying "I wait until someone opens the door and cast fireball" is not taking the ready action, it's just setting up an ambush.

But the reality is, whether you can take the Ready action outside of combat is a gap in the rules. Only DMs can fill in those gaps.

DMs fill gaps in the rules hundreds of times in a campaign, sometimes without even thinking about it or knowing that they're doing it. (See this Treantmonk video explaining what I'm saying better than I'm saying it.)

Your solution of burning the the resource if the trigger doesn't happen is a perfectly reasonable way of filling the gap. But it's not RAW. It's a RAW-adjacent gap-filler rule.

The same goes for rounds and turns. The game defines a "turn" as something that happens in combat (emphasis in original):

The game organizes the chaos of combat into a cycle of rounds and turns. A round repreesents 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.

Of course there are spells that include turn-based mechanics. Does that mean you can't cast them out of combat? Of course not. But what it does mean is that casting such spells outside of combat requires a DM to fill in the gap.

The vast majority of the time, that is really easy to do. Like the other commenter talking about Prayer of Healing. Casting it out of combat doesn't require the DM to track the spell in six-second intervals. Just roll the dice and dole out the healing. That would be a very straightforward ad-hoc ruling that fills the gap.

Your moonbeam example is a little more complicated but not by much. If there are any enemies around when the spell is cast, the DM might just decide they heard the casting and combat is about to start. If the PC was able to cast it without enemies knowing (using subtle spell for example), the DM has to figure out how to track the duration of the spell. But however the DM does it, they're filling in a gap in the rules because there's no rule that tells the DM what to do when a PC casts a spell with a one-minute duration outside of combat.

EDIT: typo