I'm not quite sure what you were hoping for them to say. They said that they could not make replying compulsory. They also said they had no facility to show when a member was last online. Maybe with your input they may consider adding this facility - but it won't happen overnight.
GR is certainly not alone in this...all the subscription and non-subscription tree sites have this problem. People come an go, some people have more interest than others. I agree it is annoying but there is not much (if anything) they can do about it.
I don't think removing old trees would be productive, because often I get in touch on other sites with people who put their trees on years ago, and never had a single contact until mine. I also don't think it would be right to remove people just because of their inactivity - people live busy lives and some may not have as much time as others to devote to family history. If it falls to the side for a couple years, they might want to come back to it when they have more time or inclination to do so.
I no longer subscribe, and I just let people who are interested contact me. I can reply and often develop good contacts. Of course, if everyone did this, the site would not produce any contacts all, but it's worked ok for me. At least I know everyone who sends me a message has a genuine (perhaps lasting) interest.
Alexander
