A trend which continues today - so by your own reasoning, it shouldn't be surprising to see the notched lapel gain favor. We wear tuxedos where once white tie was required, and where tuxedos were once worn, we now wear suits, if not shorts and a t-shirt.
So it goes. Lament the loss of formality if you want - I won't fault you for that. But the notched lapel is not a fad, but a reflection of a century's long trend. If anything, we should be surprised it took so long.
See, I'd say that the transformation from white tie to black tie is the same idea as the transformation from black tie to a suit and tie. The tailcoat pretty much fell out of use but on the rare occasions when it is worn today, it is pretty much the same garment. The fact that black tie is worn less frequently isn't really a rationale in itself for the elements of it to change.
One could have a very long and potentially interesting discussion on whether the rough similarities between a tuxedo and a suit made it easier for certain features seen on business suits to creep into tuxedos without the average man being too aware or concerned about this trend. While I continue to believe that the peak and shawl and far preferable to the notch lapel, I suspect that this played a large part in it. I buy the idea that one can see a tuxedo as a form of suit much more than the idea that one could see black tie as a form of white tie. I don't dispute that notch lapel tuxedos are probably here to stay as a practical matter. All that said, I believe that the rare occasions one has to don a tuxedo give one a chance to honor tradition in a truly elegant manner.