Spam is going to be problematic for new extensions, even those that have low abuse rates will be affected, because they will be tarnished with the same brush unfairly.
Spam projects a bad image to the public.
When sysadmins begin to blacklist new extensions at server level to get rid of the abuse, they are going to become less useful than 'ordinary' domains.
If you can't send E-mail to the outside world because mail servers reject your domain, you have a problem. It's almost as if you had leprosy. When outsiders are telling you that your domain sucks it's pretty serious because it's very unusual.
PS: I'm sure that many webforms still do not recognize new extensions. Most rely on regex validation routines that are broken. For instance IDNs are not usually handled well.