I see what you mean,I don't actually agree with you, but I see what you mean.....but you've only talked about what could happen in a team book. Like I said before this will have absolutely zero effect on Superman in his books.
Also I honestly think speed is not the ultimate power in anyway shape or form. It's a very very good power yes but not the best if you ask me. Also there are just going to be things that Flash can't do that Superman can.....like fly in space or if it's a big bad that only Superman has faced before. Powers don't decide a comic (Team book or otherwise).
Idk if your a Morrison fan but if you read his JLA (90's) then you can find a Flash that was way faster than Superman at the time.....but it didn't stop Superman from putting the moon back into orbit, wrestling an angel,with standing the soul shattering light of heaven without a problem, beating a martian, inspiring a whole planet to fight a death weapon, then destroying that death weapon.
You just have to have the imagination to use the character.....that's what we depend on the writers for. Also at this point the gap in their speed doesn't really seem that large at all and Superman can compensate for it with the use of his senses and some good old fashion brain power and creativity (JL #2....and Superman's whole comic book life). I think he's good.