What we "believe" and what we "know" are 2 different things!
We know, that the airframe will be extended for about 0.5m and that such an ammount of airframe parts will add to the empty weight. But we don't know about possible weight reductions now, that's where your believe comes in, but that hardly prove anything.
You also believe that only 1 or 2 avionics will be added, or that they will not weight much, but that's not a prove either. The cockpit alone will see new larger MFDs, it is likely that more EW sensors or systems will be added, so no matter what, the weight will increase.
Btw, it's wrong that AESA's will be lighter, since the whole radar system is much heavier even without a repositioner:
Zhuk ME (puls doppler radar with repositioner) - diameter of 624mm, weight 220Kg
Early Zhuk AE (AESA) - diameter of 575mm, weight 220Kg
Later Zhuk AE (AESA) - diameter of 688mm, weight 275Kg
All official specs, of the Phazotron specboards! The fact is, the weight increase of the AESA's is even a problem, since they often limit the size of the radar. The Russians promised to offer a Zhuk AESA with 200Km detection range, but the radar was far too heavy which badly shifted the weight balance of the fighter. That's why they had no option than to reduce it's size to the current 688mm and a reasonable weight increase. The Gripen AESA adds around 50KG compared to the older radar and both have repositioners. The EF AESA radar is so heavy, that the consortium thinks about adding weight to the rear end of the fighter, to balance the weight again..., so an AESA in the LCA MK2 will add more weight for sure!
Gripen E's IRST adds around 55Kg, compared to the older version without the system.
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So if you stick to what we know and judge that, you will see that a considerable ammount of weight will be added. You simply can't counter that by using more composites, especially not at the surface. Even if the newly added 0.5m of the airframe are fully made of composites, they add weight!
The main counter of this weight addition is the increased fuel, while only minor weight reductions can be achieved. Even the Gripen E is given in latest specs with more than 500Kg weight increase compared to the older version.