I think the stagnation should be clear and obvious.
I don't think stagnation is clear nor obvious when other characters are constantly taking 1st place over MK in their regions and in Nationals. I find it funny that Overswarm timed his political campaign the way he did; not very effective.
Besides placements characters have been growing in their attempt to fight MK by finding chain grabs, grab releases, traps, and all sorts of tricks. These same strategies that were discovered in an attempt to defeat MK also can be applied to the rest of the cast. Techniques and AT's are still being discovered.
There is simply too much room for growth and potential. ADHD has opened up a large amount of possibilities for Diddy, but he doesn't have a strong enough army to rally the support as fast as Metaknight. Still that opportunity and potential is there. The same goes for Wario, Falco, Ice Climbers, etc. It's much harder to reach that high level of play compared to MK, but it would be ignorant to say it was impossible, and ignorant to say those said players are "special" because they're not.
Evidence that would show that the metagame has become stale is when the frequent non-MK 1st placers stop placing 1st all together. Metaknight is popular and drowns the top placings, but he simply does not take 1st on the consistent basis people claim.
I mean, when I see a ZSS beating Seibrik 3-0 and getting 1st, I see the metagame growing. At this point, I see no signs of the metagame becoming stagnant in the near or distant future.