The components are required to cast the spell in the first place. IF you can't provide ALL the components required, you can't cast the spell. That indicates that in order to cast you need all of them and they are used immediately. Disagreeing with this means that you could argue that if the spell is counterspelled you didn't use up material costs, action economy costs or use up a spell slot. This is nonsense. Once the spell is successfully cast there's absolutely no more need for the components (outside of specific exceptions which will be called out) because you've already cast the spell. The components are only used for that purpose (again outside of specifically called out examples).
Lack of rules text that says you need to continue V, S, M during the entire casting of a spell including the resolution of effects for successfully casting the spell doesn't mean anything. Just like the lack of the rules saying I can't auto-kill something when attacking it with my fist doesn't mean it's ambiguous or shaky ground and that I may in fact be able to do it. Let's stick to what the rules say and not try to invent stuff k?
And I quoted not the ready text but the spellcasting rules for components themselves. Ready doesn't change how this works because it doesn't specifically state that it works differently. Specific has to be well, specific. If it doesn't say it works differently then it DOES NOT.
Spot on!
We don't have to know what the magic words for each spell actually
are, we just know that there
are some for each spell with a verbal component, and that it must be provided. If it is not provided, the spell is not successfully cast.
Let's say that the verbal component for, say,
lighting bolt, the magic words which must be provided, is the phrase "So long as men shall breathe, or eyes shall see!"
If you just say "So long as men..." and then stop, have you provided the required verbal component? No. Does a partial bolt of lightning, roughly a third of the usual length, fry some of the baddies, with the bolt growing longer as you continue the phrase? No.
How do we know? Because if you just said "So long as men..." and the spell effect, or part of it, appeared, and then you refused to complete the phrase, then the spell effect appeared even though you never provided the verbal component! You can't just go around shouting "So...!" and expecting your spells to work anyway! You've failed to provide the verbal component (the whole phrase) and so, RAW, the spell fails.
This is why we
know that the components must be fully completed
before the spell effect begins, because until you complete the components, you haven't provided the components.
When the Ready action lets you ready a spell, then the wording that lets you fully 'cast' the spell as an action (the components/spell slot) and
later use a reaction to release the spell effect, relies on the already existing spellcasting rules that mean the casting process must complete before the spell effect begins.