I have had this discussion repeatedly elsewhere where where would-be-experts tell people to buy L series lenses for their Canon cameras because they are the 'best'. When I enquire as to why they are best I get told things like 'they are tack sharp right up to the corners' (an actual response I got). My point was - if you look at your photographs and think they are not good enough because they are not 'tack sharp' in the corners, then you need a Canon L series lens. Personally, I have never thought that.
I don't worry about lenses too much - I am not overly bothered by technology at all - but I sell more pictures taken with a fifty-odd year old Tessar than I do taken with my expensive modern (not L series!) lenses.