You must be a young fellow to think that a lens that came out in 2014 is an old lens! While the original EF 100-400 has been reported as being less successful in taking advantage of higher MP cameras such as the 90D, there should be no problem with the Mark II. Perhaps AlanF can verify, but I think comments from others have indicated that the EF 100-400 mark II is just as sharp as the RF 100-500. Someone please correct me if I'm wrong, but I think you'll be able to use this lens for the next 20-30 years with no problem.
Most of my lenses came out before 2005, so 2014 is quite new in comparison. However if you buy any technical product and it came out eight years ago, that already seems old, even if I of course want to use it for decades into the future. When I bought my 70-200 f/2.8 IS (the first version) though, I also thought that the lens might be good for decades, but compared to other 70-200 lenses today even from Tamron and Sigma it does not really keep up. As long as I use cameras below 25 megapixels or so, that is hardly a problem, but if I had an R5 for example, the 70-200 would be high on my list of lenses to replace, because it simply is not sharp enough to really support such a resolution. Another lens that is far from today's standards is my 24-70 f/2.8 (also the first version). The corner sharpness does not seem acceptable to me today and it does not even have IS. Basically ANY other 24-70 option you can buy today is much better than the famous Canon lens, although I still love the fact that it retracts when zooming in, which gives the lens hood the perfect lenght at every focal length. So that is another lens that I thought I would buy for decades, but after less than two decades I hardly use it any more.
Today lenses are designed by simulating billions of light rays from all directions and in all visible wavelenths. That's one of the main reasons why lenses got so much better in the last two decades. That simulations require an enormous amount of computing power, because for zoom lenses they have to be done for many focal lengths and focus distances. So progress in processing power of computers should result in better lenses. If a lens came out eight years ago, there was a lot less computing power available compared to today. At one point in the past or future though computing power should no longer be limiting factor. Maybe that point was already reached a few years ago. That would mean that future lenses would no longer benefit from faster compueters.
As Canon is infamous for crippling its cheaper cameras, I really wonder if Canon makes sure that the cheap options are sharp enough, but not "too sharp". What if a simple optical formula that could be build for a low price turn to be so sharp that it could cannibalize Canon's super expensive lenses? Isn't it quite odd that Sigma and Tamron often offer sharper options in the low and mid range price range than Canon?
