Because of MMR, there will be quite a bit of mixing going on from season to season. Guilds that finish 4th or 5th won't see too much difference in their level of competition from season to season*, but guilds that finish 1st or 8th are going to see pretty major differences. A guild that finishes 1st will have its MMR adjusted upward by 175. And a guild that finishes 8th will have its MMR adjusted downward by 175.
Take a guild that has an MMR of 250. Solidly in Silver league. It wins a close battle and takes 1st. Now its MMR is 425 - Gold league. But who is it facing? A guild that was at MMR of 600 (not too far away from platinum league) but did terrible and took 8th place so its MMR is now 425. These two guilds were 350 MMR apart from each other the previous season and now facing each other. For one, it will be a huge challenge. For the other, it might be a piece of cake.
The intent of the MMR system is not to match up guilds that are close in abilities week after week. If a guild does well, it will be facing tougher competition. If a guild does poorly, it will be facing easier competition. From season to season, the standings get scrambled up quite a bit. This will provide almost constant variety.
*And how about those guilds that finish 4th or 5th? Well, they aren't just matched with other guilds that finish 4th or 5th. A guild that was at 400 and finishes 4th is now at 425 and on the same battlefield as those earlier two guilds. Similarly, a guild that was at 450 and finishes 5th will also be at 425 and on the same battlefield. Any battlefield will contain a wide mix of guilds, some that did well, some that did poorly, and some that did so-so.