When is the last time you cleaned your camera's sensor?
Yes, it's possible for a lens to "go soft" by losing calibration over time with wear and tear or if it were bumped and an element got de-centered.
More likely would be gradual accumulation of dust and other "stuff" on your camera's sensor, increasingly degrading resolution.
Even more likely would be some slight camera shake blur effecting one of the images.
Also, are you making a fair comparison? Frankly I'd expect a $1500 L-series lens to out-resolve a $400 non-L.
However, I really don't see much difference between the two 50mm images shot 6 months apart.
One solution... stop looking at 100% crops. At least don't use that high magnification to evaluate sharpness. If, say, you have an 18MP camera, viewing it's images at 100% is like making a five foot wide (or tall) print and evaluating it from 18-24" away. You normally wouldn't view such a large print anywhere near that closely. Back off to 35% for more reasonable evaluation. Even at 35% an image from an 18MP camera is equal to approx. a 16x24" print.
Your test shot of the focus target is significantly underexposed, too... which I often think gives the impression of softness, too.
Plus, the test shot with the focus target is at f1.4 (which is widely known to be softer in the EF 50/1.4), while the two dog shots are done at f3.5, which is a much sharper aperture with that lens.