We should not forget that 12-15 years ago, there was no alternatives to Leica M-mount lenses. You bought what you needed (or wanted) and used it. Sometimes used and sometimes new.
With the arrival of the Konica M lenses, the Voigtlander Cosina lenses and later the Zeiss M-mounts, we suddenly had alternatives, and often at more palatable prices.
I have bought a fair bit of M-lenses new, just because I needed them - but I have also gotten rid of a lot of lenses by Leica when I found alternatives by other makers that did what I wanted them to do at much less cost - and in some cases better performance too.
At the moment I have 35f2 Asph, 50 Lux Asph and 75f2 Apo Asph and the new Elmar 21f3.4 - all bought new - but at prices that in most cases are 1/3 to 1/4 of what they are today. I would not have paid $4000+ for a Summilux 50f1.4 or the 75f2. However good they are, they are not THAT good. The Summicron 35f2 Asph was bought new when it came out and I have kept it because it is my "reference" lens for f2 35's in terms of sharpness etc - but I cant say that I am enamored with it. Fairly unpleasant "jagged" look to it in bl/w and vastly overly contrasty with color.
The Elmar 21f3.4 was one of those items that I had to have. Having been "attached" to the Super Angulon 21f3.4 for more than 40 years. this was more of a sentimental purchase. Is it as good as a SA 21/3.4 - yes, in many ways it is and in terms of performance at f3.4 better - BUT, I could happily have survived without it.
The 50f1.4 Asph is now "silly" priced. Yes, it is probably the best 50 mm lens ever made - but what would you be shooting that made you need that! Portraits - no way, nobody looks that good. With film you have to use 25/50 and maybe 100 iso to see it, and I find that I grab my C Sonnar 50f1.5 more often than the Asph 50f1.4.
As to justification, it is personnal. If I have the money floating around, yes - it is nice to get those silverboxes with neat padding etc - and those leather cases smell nice.
It is also nice to know that the equipment one uses is better than the skills one has. Cant ever blame the glass - as if a client would care!
I shoot black/white, normally a 320/400 iso film and there are few shots I have taken where the lens was the deciding factor in quality!!!
The optics we have available now are in most cases so good that the quality of the image is more depending on the users skill than the camera and/or glass.
Today I wanted to finish of a roll of the ORWO 74 (400 iso film stock) and I took a LHSA M6 TTL and the Summicron 75f2 and an hour walk. I thought about this as I was walking around snapping - it is a package that would cost me $6-7000 to replace if it got lost! About 4 times what my car is worth! I could have taken a Bessa R3M and my Heliar 75f1.8 or even the 75f2.5 and most likely could not have found a major difference between the images.
Ok, once you have the 50f1.4 Asph and the pain of paying for it have subsided, just use it. That what it is for.