I am mostly basing my understanding on this video https://www.youtube.com/watch?v=RQv5CVELG3U (I also have a background in lasers, and a tiny bit of undergraduate QM but nothing too profound)
I can understand why the interference pattern is destroyed when we measure the photons using detectors D1 or D2 in this diagram.
The detection of an entangled photon at D1 or D2 gives conclusive evidence regarding which slit the entangled photon took, so there will be certain that there will be no interference pattern. Similarly, the distributions of the particles associated with the "erasure" detectors D3 and D4 (shown in the video but not the image above) are completely reasonable to me.
My question is, what if there are no detectors at all? What if we just let all the entangled particles going towards the right side diagram just travel off to outer space in a way that no one can ever detect them? Or put a crude black obstruction that would absorb the photons in a way that no one can recover the which way information?
From the sources I read, I get the impression that no matter what you do with the entangled photons going to the right side of the diagram, the total pattern on the screen will be the sum of two non-interacting single slits - in other words no interference.
This leads me to conclude the "crystal" placed after the slits is responsible for destroying interference. The way I see it, the mere existence of that crystal after the slits is profoundly altering the wavefunction of the photon. As soon as the crystal "works its magic", any hope of interference is forever destroyed right then and there. (How accurate is this statement? I am talking about an interference pattern created by all the photons. I am well aware that if we select a subset of photons by an informed matter, for example using detector D3, we could see a pattern.) What is this crystal doing, and why do so many explanations completely omit the seemingly crucial part it plays?
A follow-up question: What if we used a laser that generates many particles, rather than a single particle generator? The right side of the experiment still goes off to infinity where they can never be detected. Do we expect to see an interference pattern in this case? I see no reason the laser case should be different, so no interference. But if we do not observe an interference, will it even be a "laser" beam coming out of the crystal?
I asked many questions, and please feel free to engage with whichever ones you think will be useful for clarifying my misconceptions.