I'm not sure why the OP isn't considering USB external hard drives for backup purposes, because HDDs have a much better lifespan than the flash that you can find in a USB pen drive.
Probably true, unless they are made by some unknown manufacturer with cheap components.However, if you want to leave something for your children or grandchildren I would still choose physical backup of seed words on stainless steel.
There is a good chance most drives will become obsolete in future, in same way like VHS and CD drives are now.
But in terms of lifespan, M-Disc would be better option as it claims can last 1000 years with bigger storage. Although you need to encrypt the file manually and certain DVD writer which also support M-Disc writing.
I never in my life saw M-disc and you would probably have hard time finding and playing it in future, but yeah they will survive longer unless someone scratches them 