Someone on another thread posted another possible reason why people with private trees doin't respond to emails. They can look at your public tree even though you can not see theirs and may have already decided a match isn't readily identifiable (a mistake because it mighthave been possible to work together to figure out the link as i have done several times with public trees where the linkwasn't readily available).
Which again makes me think this problem would be greatly reduced if we had the option to send notice of our DNA match only to people with other public trees. Or an option to just send private tree owners the same kind of notice we now get when we match them (i.e. user name, size of tree and note saying click here to contact this user who has asked we share their tree only with other users with public trees). That would then require them to reach out for access as public tree users must now do with very limited information. Surname list for all matched trees would be another approach.