In the earlier days a plastic mount entry level SLR camera took the SAME photos as a top of the line $2000 professional SLR camera like the Canon 1D.
Now why did people bother spending more for a better SLR camera when the film (just like a DSLR sensor) was the same for both cameras.
The answer was build quality, performance and ergonomics.
The same thing applies here as well. If you had used your same line of thinking that photo quality was all that mattered then you would have been very happy with a entry level SLR. However many people still went for more expensive cameras to get a metal mount, more buttons, better viewfinders etc. All these features added nothing to the final IQ.
In this scenario, sharpness is just slightly better than the f1.8 version but build quality, speed (2/3rds brighter lens), AF are all better. Again none of these contribute to a better photo and you (and many more people) feel the price difference is very significant that you do not care for the small advantages of the f1.4.
There is nothing wrong with that but do remember that there are also many other people who do find the advantages of the f1.4 version to be worth paying extra for.
Also do note that a well made lens can last you a lifetime. You get many fully working 1932 era Leica lenses still functioning perfectly today. However the all plastic (down to the mount) 50mm f1.8 may not last nearly as long and even less if (God forbid) it falls to the ground !