I don't really do much dressage... but flat work is fine as long as we don't do the same thing for to llong.
I <3 jumping, though.
If you want to jump, you have to have strong fundamentals in dressage.
GP horses do minimally level 3 dressage.
Afterall - stadium jumping is dressage with speed bumps.
If you don't have a well rounded horse, working off of their hind end and light on their front, if you don't have a horse with no lateral work, no balance, no rhythm - you have nothing to be successful in the jumping ring.
You have to have dressage to have rhythm, tempo, fluid movement, a light horse, lead changes, balance, roll backs, striding, extending, collecting, a rounded horse instead of a heavy flat horse, straitness, relaxation, softness, suppleness - and a horse who can rock onto their hind end with ease for those bigger jumps.
Without it - you will not be successful in the jumping ring.