Somebody says this with every major pixel jump and every time they are always only marginally correct.
It happened with 8MP to 12MP, 12 to 18, 18 to 22/3, etc... "The lens can't even resolve that!"
Sure, some lenses, particularly consumer and mid-grade kit lenses (like the 28-135mm my 5DmkII kinda makes go pear-shaped everywhere but smack dab in the middle of the frame unless you stop down to f8 or so) will offer no additional resolution on a 30-50MP camera simply because the sensor out performs the lens. But plenty of lenses still more than out resolve our current sensors...and as history has shown, at least with Canon, any L series glass that doesn't perform and is in mainstream use will get updated within a few years.
I can say personally that even my current 18-23MP camera stable occasionally fails me due to lack of sensor resolution. Usually when I need to crop to 5:4 dimensions and make a large print. Only with one it two of my lenses do I run into resolution issues, and they're consumer lenses I use to full gaps in my L kit, or use due to their lighter weight/lower cost in more gear-risky environments.
I'm eying that 50MP 5Ds with some "maybe in a year or two" coveting. A lot of what I do is either with very high resolution medium to long primes (macro and wildlife) that have a lot of resolution to work with, or requires just enough cropping for 5:4 prints that I'd prefer a few more megapixels before making the chop. I remember the change from 8MP up to 12MP, and then when I went from 12 to 23MP, and the ability to crop and still make a print I was satisfied with increased dramatic at each step.