Apart from sponsorship and jersey sales, ticket revenue is indeed the main income for football clubs, for rich clubs like Bayern Munich and Dortmund it may not have too big an impact on them, but for clubs that have financial imbalances it will certainly make them limp in following the competition later, I think that apart from implementing the rules without spectators, the league board should also anticipate the possible effects on the club later and maybe what is done in the premier league when covid spreads and forces matches without spectators can be imitated by the German government by establishing the Covid Corporate program Financing Facility, ie clubs can borrow funds for club operations with a small interest percentage.