I'm a little late to the discussion regarding a revived Elder Club, but I wanted to leave my $0.02
I actually think an Elder Club would be viable and not terribly costly to UFP. The original Elder Club was mostly active with touring and was a way for H!P to get bigger venues for their artists who couldn't fill a hall on their own. In any case, it was a way to retain a girl's fanbase within the H!P sphere without having to necessarily give them new material all the time like the main acts. Yaguchi and Satoda did variety very successfully while still participating in summer and winter tours. A lot of the members did stage plays during the off season. They definitely earned their own keep and retained the benefit of exposure through H!P events.
Contract renegotiation would definitely need to happen, since UFP has always been a fiscally conservative company, but it would prevent fans peacing out altogether when so-and-so graduates. It would also give these new idols some veteran members to help teach them the business and what not to do. For members who want to stay in the business, like Tanaka/Airi/Harunan, I can see it working out really well. Management has proven they aren't averse to keeping a member under the H!P banner - even if only in name (like Aika and Riho), but it would need more than a few members to do activities like the Elder Club did.