If you want an absolute answer to this question, you'll probably have to test it yourself using hundreds of blank media, and waiting years for the answer. Since this is not likely to happen, you can ask yourself this question: "Manufacturer A sells expensive media and have done some tests to simulate aging of discs. Manufacturer B produces cheap media at the lowest possible cost. Which media is likely to last longer?". You won't know the answer to this question for sure until media A or media B fails at some time in the future. Do you want to save money now, or do you want to pay more without knowing with 100% certainty that the more expensive media will last longer? The choice is yours, and others may choose differently than you.
This type of question is not specific to buying media, and I don't think you would normally be able to know with certainty that an expensive brand you buy will last longer or be "better" in some other way than a cheap brand.
Very little can be known with 100% certainty in this world, so why do you expect to have this certainty when it comes to blank DVD media??
Nobody knows how long it will take, but it will never improve compared to the day it was written - it will only stay the same or deteriorate.
What is normal use? I for one certainly don't scratch my CDs and DVDs on purpose, but they do get scratched anyway on occasion. Are you seriously suggesting that you have never had a CD or DVD become unreadable or damaged? Well, for (some) other people this does happen, and it's not even necessary to use the discs as frisbees or have a kindergarten full of children play with the discs for this to happen at some time.
Discs can also become unreadable over time just sitting on the shelf somewhere, although this is a rare occurence. It has happened to at least a handful of CDs for me, and at least one of the discs were thought to be of excellent quality (Kodak "gold" CD). I have only been burning DVDs for a few months, so I haven't personally experienced this problem with DVDs yet.
The quality of the disc will NEVER be better than the day it was written, so if you start out with a barely readable disc, do you seriously think it will be able to withstand wear and tear as good as a disc that was "perfectly" written? ...I don't think so! ...but I'm not going to try and prove it either.
Also consider this: If you burn a CD/DVD in your burner and verify that it can be read back without any errors, but you don't know what the PIE/PIF error rate is. Will the disc be readable in another (inferior) drive? Will it still be readable in your own drive when the optical pickup deteriorates over time? Without having some kind of quality test in addition to the Verification Passed/Failed test, you have no clue as to whether the disc is perfect or only barely readable.
One could argue that it shouldn't be necessary to worry about these issues - all burned media should of course be readable in any drive at any time in the future. This is unfortunately not the case as numerous posts in this and other forums will testify to.