For the most part. I wouldn't play it against the English though.
1. …d5 reasonable response to any opening by white?

Thanks. You are probably right. That would typically lead to 1.c4 d5 2.cxd5 Qxd5 3.Nc3, I guess, and black have to retreat the queen and white have better development.

1. e4 d5 and you have a Scandinavian Defense.
1. d4 d5 and you have a standard double QP opening.
Even against the English, 1. c4 d5 2. cxd5 e6?! because why not? Have fun. Just don't play 2...Nf6 3. e4 Nxe4? 4. Qa4+
Ouch!

Thanks. It is good to some simple rules of thumbs to be able to play faster, I think. I did not consider 2. cxd5 e6 – I’ll study that variation.

If you want to study one system for everything then go for 1...c6, you will end almost all of the time with a Caro-Slav structure, which should make study easier.

Mmm, no.
1...e6 leads to the QGD or French depending on White's 1st move.
1...g6 leads to tons of possibilities
1...c6 will almost lead to a Caro-Slav structure where Black has pawns on c6-e6 and White has a pawn on d4.
Ofc 1...c6 is not a magical solution, White can play the Advanced Caro Kann, or the Exchange, or the 2 knights, but it will usually lead to more Caro Slav structures than any other opening, which could help minimize study.

Agreed. 1. ... d5 leads to a decent opening against every White move except 1. c4, and even there it's marginally playable (but inferior).

Caro Kan and Slav are actually very different so it is a mistake to talk about Caro-Slav pawn structures.
So now all pawn structure books are wrong for talking about the Caro-Slav complex?
They have their differences, specially in the pawns that are exchanged, but the plans for Black are usually the same, break with c5-e5 or target d4 if his sister pawn has advanced.
Still, there are differences, and I clarified those in my posts.

Caro Kan and Slav are actually very different so it is a mistake to talk about Caro-Slav pawn structures.
So now all pawn structure books are wrong for talking about the Caro-Slav complex?
They have their differences, specially in the pawns that are exchanged, but the plans for Black are usually the same, break with c5-e5 or target d4 if his sister pawn has advanced.
Still, there are differences, and I clarified those in my posts.
But these are the plans in Queen's gambit accepted and declined and in French Rubinstein.So why don't you call them Caro Slav French Rubinstein Queen's gambit pawn structures?
Maybe because the name is too freaking long? What does that has to do with the fact that if he plays 1...c6 he will usually get structures where he has to do just what I said, play c5-e5, or restrain and attack the d4-pawn if his siter has advanced? Am I wrong in some way?

I have the following rule of thumbish hypothesis: Playing as black 1. …d5 is a reasonable response to any opening by white. I am however a mere beginner, so it might very well not be sound. What is your opinion?