Most of our sites have been accepting guest posts for ages. We have many members who grew together with us (they joined as very new sites and are now getting traffic, rankings and activity), so your observations are wrong.
None of our oldest and most established bloggers has been devalued so far, so I don't see any point in getting suspicious.
New sites are more active, true, just because they are new. So they might be easier to notice.
We are not doing anything wrong and saying "after MBG sites are all devalued" is also wrong. Let's talk about that if/when it ever happens. We have worked hard to be Google-friendly. We are doing our best to convince the members of non-link-building priorities. And most of our users do get that.