The trouble is that if the rules were to be read that way, since the Attack Action itself only allows one attack (it does), then 'grapple, divine smite, BS substitution, and maybe even sneak attack' can never be used ever!
No, the attack action states:
With this action, you make one melee or ranged Attack. See the “Making an Attack” section for the rules that govern attacks.
and then it notes
Certain features, such as the Extra Attack feature of the Fighter, allow you to make more than one Attack with this action.
The word "only" is not there. It simply does not grant the ability to do anything else. There is a difference between "banning something else" and "permitting something that does not include something else" in English.
The specific haste restrictions to the Attack Action only restrict what they say they restrict: only weapon attacks not spell attacks, and only one attack even if you would normally get more.
You added "only" to the rules of the attack action then removed it from the haste spell.
The Attack action permits you to make one attack. It notes you can make additional attacks with other features.
Rules then talk about substituting that one attack for other things.
The haste action states you can make an attack action consisting of "only one weapon attack".
You can read this as "use the attack action rules, but you are not allowed to use extra attack, and the attack must be with weapon". That is
one reading of what it says.
Another reading is "during the attack action, you can do no more than one weapon attack". Another reading is "
Haste does NOT have other restrictions you make up, even with a sideways squint.
It says
one weapon attack only. Your reading of that is only one possible reading.
Go into the store. You can buy one chocolate bar only.
This could mean (a) you can buy whatever you want, but no more than one of it can be a chocolate bar, (b) you can buy nothing but a chocolate bar, (c) you can do nothing in the store besides buy one chocolate bar.
The English language doesn't actually distinguish between those 3 meanings.
Whichever meaning the sentence "One weapon attack only" is (and there is more than one) is the restriction on the attack action you are allowed to take. If you fail to meet that restriction, you are not permitted to do that in that attack action. What that restriction is depends on how you define the meaning of the phrase "One weapon attack only".
I get your model. It is a reasonable model of how to adjudicate what haste does. Congraduations. It is by far not the only reasonable model of what those rules mean.
If you disagree, repeating your interpretation doesn't really help. You have to prove not that your reading is reasonable, but that all other readings are unreasonable, and you aren't doing that.
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Suppose it said:
1. "You can do anything in that attack action, so long as you make no more than one weapon attack".
2. "This attack action can consist of doing a single weapon attack and nothing else at all."
3. "The attack action cannot benefit from any extra attack feature to get any additional attacks, and the only attacks you can do are weapon attacks."
All of them are
reasonable readings of the phrase "One weapon attack only"; I can produce English sentences where "One X only" clearly has any of the above 3 meanings. And if you replace "One weapon attack only" with any of the above 3 phases, only one of them results in your reading of how the rules work.
