Can you argue that the main reason is that one? In the past fleets were also lost and the game was much more alive.

maybe I will increase repair only down a point limit or before N weeks old account

there already is something you suggest:

OGame 6.2.0 - Let the "wreckoning" begin!

Additionally, the way this feature exists at the moment is rather overpowered as to what the defender is able to recover after the battle and basically allows to restore 1/3 of the initial fleet where balance-wise that amount should only be 1/4 or 20% less than it now is.

make it 30 minutes then I say
good suggestion

the problem with such a high rebuild percentage would be that you could create ressources from nothing. 70% df + 50% fleet means more ressources than there were before
the wreckfield suffers extremly from the fact that every second server now has like 70 or 80% df because it means defenders can only get 20 or 10% back
fleeting with 30% df kinda sucks but 70% is too extrem in my opionion.
I think 50% df for every server would be ideal especially considering new features like the wreckfields who would be easier to implement and balance with set df%

amount of recostruction must linked with amount of debris f that universe, to prevent that sum of debris + rebuilt give more resources that before.

How i wrote before , i think that improve amount of rebuilt fleets can be good only or noob players not for everybody (otherwise we will have crasher angry because often seen they are happy to see defenders broken)