Given the ship was lost in 2278, and the hull configuration the hull wouldn't be very old 10 years at most. So there would be no reason to retire a hull farily new even if the design itself is 100 years old.
Case in point: Enterprise-A
Commissioned in 2286, decommissioned in 2293.
Sector 001 by the way, is Earth. It wouldn't be too hard to figure that this 100+ year old ship would be relegated into the role of a training ship in and around Earth's solarsystem. Since the battle took place near Earth, it makes perfect sense for ANY ship within range to participate in the battle, even if it's potentially a "no win scenario".
"higher than Warp5 is emergency only or they can damage subspace"
Well, since the Intrepid Class introduced the "variable warp geometry" style pylons/nacelles specifically as a workaround for said problem, I doubt that any newer ships would have a problem with "too fast speed, damage subspace" if the old Intrepid Class didn't have the problem. While no other canon ships have been seen with a similar pylon/nacelle to the Intrepid Class, it's likely they made similar modifications to the ships. The "Warp 5 limit" has been broken time and time again since that episode of TNG, both during emergencies and just general warp travel.
Bottomline is: Your assumptions are just as valid as mine. We may very well both be wrong in our assumptions. Until there is anything that can realistically disprove the assumptions, it's just a bunch of speculations. Think this topic should try and slip back into being "on topic" rather than speculating on the life history of different ship classes.