This is a case of unrealistic expectations. This is also a case of using the internet as a trump card to complain when it would be useless for the boutique to offer a rebuttal.
You saw something that wasn't the original strap. You bought it because you liked it at the time. You changed your mind a few days later. Who's mistake was that? Why should the Lange boutique eat your mistake?
Put yourself in their shoes...
What kind of business are you in?
If you sold something in good faith and it was used, would you take it back?
What kind of precedent does that set?
What if all your customers did the same thing? Is that sustainable?
You never mentioned you bought watches from that actual boutique. You somewhat lead the reader into assuming that you do because you have a few Lange watches, but you never say you've patronized the boutique for the watches. Not that this matters. It's irrelevant.
I think you've been treated well, gotten too accustomed to being treated well, and are complaining at the slightest thing when (in my opinion) you are in the wrong. You're unreasonable (in my opinion). You expect way too much in terms of being catered to your whims. And when things didn't go your way, you kind of retaliated by complaining here where the boutique can not rebut. I state these as facts of observation. I don't know you, it's not personal. We haven't met.
Let's give a parallel scenario.
1. I buy a lot of cars from a dealer.
2. There is a new SUV I want, and I don't know if I would want it in the flat black or metallic black.
3. I explain this to the dealer. Dealer may say one of two things "wait until you see the flat black before you make your decision" or "I'm sure you will likely love the metallic black as much as the flat black." Regardless of either line they say, it's irrelevant.
4. I buy the metallic black one without seeing the flat black one.
5. A day later, I see my neighbor/colleague with the flat black one, I decide I don't like this metallic black and I want to get the flat black one.
6. Another day later, I tell this to the dealer.
7. Another day later, I drive the car in expecting it to be evenly traded.
Substitute "car" with other things. Shoes, clothes, etc.
Do I expect the dealer to trade the car for me?
1. Some would say, "But I'm a super VIP customer!" That's irrelevant.
2. Others, "But I send back bottles of wine and food at restaurants all the time! How come you don't do it." In many provinces/countries, that is a legal requirement, you are not obligated to pay for food you announce as substandard and will not consume.
3. "I'm sure your competitor next door would do the same and I'm not even a super VIP there, why are you so inconsistent with the marketplace?" "Our establishment isn't on the same bandwagon."
4. "You clearly have made a profit from me. Why can't you take a small loss on me this time?" Not sustainable if all of our clients wanted to do this.
Sometimes, there are things that are really hard to make a decision on. And there should be consumer protection on these things. But often there isn't enough consumer protection.
1. Tires. I have no idea how the car will perform with those tires. Also, tires behave differently as they wear. They're good for the first few thousand miles, and then sometimes they can become loud/harsh.
2. Cell phone coverage. The phone works well at the store, but does it work well at your home and office? Sometimes some phones don't work well at my home, but by the time I take it home, it can sometimes be too late as the contract is in place. It is costly to cancel the contract even though some contracts allow a short time for low-penalty cancellation. This high cost to cancel is because you've already made an investment in the hardware and the contract doesn't necessarily reimburse you fully for the hardware (phone).
Many others...
To our member "learner" I hope I don't come off as too harsh. But put yourself in their shoes. If you were in the boutique's shoes, you would find the customer downright unreasonable.
Also, you take a considerable risk by posting this here. Which I don't think you've thought out too well. Others like you will side with you, but present this to a judge/jury of "normal people" then I doubt you'll be supported. I wouldn't expect the invites anymore. The boutique would say you're a high risk client, "Burn me once, shame on you. But I won't give you the second chance to burn me twice."
Anyways learner, I hope you learn from this.