So modifying the skyjump-angle to whatever is optimum, can we elongate the carrier making those runways longer, to make jets haul more payload?
It can't be deduced as simple as that; As I said earlier, there is a reason that Naval fighters carry lower load with their land based counter parts or with heavy modification. If you want to launch an aircraft with increased load and don't expect it to land on the deck again with that load, then you may be right. Let's get a little pedantic about science of ski-jump launch;
h is the minimum clearance above the sea level required for safe operations; which is kept constant irrespective of load-out.
This is the force balance of the aircraft launching from ski-jump. Following points are to be considered w.r.t your question;
1. Increasing length of deck means increasing the speed of aircraft at the point of separation from the deck assuming aircraft is on maximum thrust. But here, point is kept in mind that the lift 'L' becomes significant much after the separation when the aircraft is appeared to be dipping yet its speed in longitudinal direction is increasing due to continuous thrust.
2. The increasing longitudinal speed will increase the lift balancing the weight in the vertical axis and the aircraft will start its normal climb.
3. Again, the iterations could be done for longer airstrip, in fact, reaching up to the point where it doesn't require ski-jump; but the economics of building such a long carrier will be be gone for toss.
4. This is the precise reason, why there is a lot of emphasis on T/W ratio of aircraft for considering carrier operations.
5. With increasing rated load, you have to strengthen the undercarriage and belly structure as well; that puts additional weight penalty on the carrier fighter. Moreover, the fatigue load experienced during landing will increase as the weight increases.
For more details, please refer to this journal article;
As I said earlier, the carrier length is optimized much more for carrier operations which also involves successive launching of multiple aircraft in short duration; landing or approach to landing; accommodating more aircraft on deck, and many more.. It's a complete science in itself !
So the long & short pathways for take-off that you were indicating in the figure are for facilitating successive launch of aircraft IMO; you can get the idea by locating the thrust/blast deflectors positioned in that path,