It's not about MERN vs MEAN. Both of the stacks use the same technologies with one exception. MERN uses React in place of Angular. But that doesn't mean MEAN is outdated and MERN will take over. There is place for both React and Angular. It's a matter of preference and what suits better for a particular use case. Some like to use MERN and some others prefer MEAN stack.
Can MERN be a successor to MEAN? No, both will co-exist. Is MERN a popular tech stack these days? Yes, it is and it'll gain more momentum in coming days.
Having said that, I would like to mention that we work really hard to enhance MERN and release new versions. It's a long term investment for us. So, be assured that it'll keep getting better and receive updates.
Hope this helps.