People underestimate the amount of technical skill and development it takes to create a lens.
When you start adding "breathing", and other video considerations... the cost quickly moves beyond the pro/consumer range that is Canon's bread and butter.
Why people feel the need to "update" their glass when a new version comes out is beyond me.
Great lenses, should, last you your entire career if taken care of properly.
For example.. I just purchased a 28-70mm L about a month a go...
It's rubbish on a crop sensor...because it was created in 1993...before digital.
But now... on my 5d3, its incredible!
Canon went from having to update lenses for digital in the early 2000's, which meant lowering the quality a bit because lenses couldn't be designed soley for one format...(FF etc), to now having to re-evaluate their quality to match the ever increasing sensor technology.
I decided to hedge my bets (and save money), buy getting some well kept "old School" glass, along with some of the new hotness.
Here's one (albeit small) from the 5d3, and the aforementioned 28-70 2.8L