The thing is, you can't (and don't have to) improve EVERYTHING. Hypothesis:
VF-25 is faster, better weapons, better radar, stealthier, longer-ranged, easier to fly, easier to maintain, easier to build (and thus easier to supply the colony fleets)---BUT is a bit less agile than the VF-19S. Is it a better plane?
How many things will you trade agility for?
Just because something is newer/better, doesn't mean it's going to surpass everything in every way. Look at the F-22--it is not the fastest, not the longest-ranged, not the highest payload fighter of all time. Yet it's still considered the uber-fighter due to being so good in other areas.
And look at the F-35. Slowest US fighter in 30 (or maybe even 40) years. Less agile than a 30-year-old F-16A. Pretty sad payload and range IMHo. Yet still considered overall better than anything but the F-22 (and maybe Super Flanker) due to its stealth, radar, and datalinks.