explicitly specifying the domains you want to relay mail for is a security measure against spam and other types of misuse of smtp systems.
if you are only interested in automatically enabling relaying for subdomains, you can set parent_domain_matches_subdomains variable.
postfix doesn't support blind relaying ... at least to some extent. in some scenarios, you may use permit_mx_backup to relay for domains that your mail server is listed as a backup (secondary, higher dns mx rr priority) mx. but do carefully read permit_mx_backup specification and related documents like permit_mx_backup_networks. but this is very dangerous and I already see ways of exploiting it via dns. don't doit.