When you stop to think about it, the fact that the people that were on Tenrou are apparently weaker relative to the average wizard makes absolutely no sense. Why has the world in general become stronger?
Over 7 years each guild would have people joining, retiring and ageing, thus the relative strength would remain the same, unless there’s an outside factor. Maybe some of younger the characters, like Max, would benefit from a few years growing up. But if age = power then Natsu would have always been among the weakest in the guild.
There’s no way Erza could go from striking fear in the hearts of Dark Guilds everywhere to having people doubt her strength, unless everyone is either stupid or has amnesia.
Also, on a side note, how on earth did Fairy Tail become the weakest guild in Fiore? Max alone is, at the very least, as strong as Natsu was when he defeated Erigor. And Eisenwald at that time was considered fairly formidable, so surely Fairy Tail ought to be viewed as at least as strong as they were? Did they never take missions or something?
Sadly Fairy Tail seems to make these kind of oversights quite often, like Lisanna apparently being stronger post-Edolas despite presumably not using her magic there. Its on of the many reasons that One Piece will always top Fairy Tail for me.