Some things that you've neglected.
1. It's probably a long-term move. I doubt they expect everyone to switch overnight. If they continue this trend into the next expansion when there's effectively a progression reset, it will have an impact on peoples' decision-making process.
2. Many guilds have multiple 10-man teams. My own guild is one such example; we actually have 3 10-man teams. Merging multiple 10-man groups (and adding a few people or subtracting a few) is another way to form a 25-man group and preserve progression. This change might mean we're going to merge our groups and get some better loot.