1) Who are "many top players", and what proportion of the players does this represent?
I cannot give exact figures because I do not recall exactly all with whom I have talked or of whom I have read something. Typically it means: I have seen only at most one top European (current supergroup members) player with a contrary opinion.
2) Why should the system be redesigned because the top players are unhappy with it?
To make them happy. They know from their own and relevant experience what matters for a good system.
2b) Who is the EGC designed to serve?
I assume that you mean EGC = (Closed) European Championship.
Your use of present tense makes it necessary to distinguish between current system and possible future system. I explain a bit for the latter:
- Those top Europeans wishing to find out the currently strongest among themselves.
- Those top Europeans wishing to have interesting games against each other.
- Those observers wishing to enjoy the games, the event, the players, the winners.
2c) Does this include non-Europeans?
Given that I assume you to mean EGC = (Closed) European Championship and further assuming that it should have European-only games: no. (Except for non-Europeans as observers.)
3) What positive aspects of the current system have been considered that it is equally important not to lose when implementing a new system?
Besides those mentioned earlier? None. For those wishing the EC to be separate, aspects like a) letting top non-Europeans play top Europeans during the morning or b) having a single tournament with a continuous players field from bottom to top are considered less important.