I currently raid 10man because there are no competative 25mans left on my server (except 1 polish only guild), many people are in the same boat as me and alot of my old guildies went x-realm to continue 25man raiding. I would happily sacrifice progression to return to 25man raiding, there is a much greater sense of accomplishment defeating a boss as a bigger group and many more logistical problems can arise such as correct formation of ice blocks on sindragosa (randomly picked an example out of the hat) I see no problem reverting back to the old system providing these conditions are met-
A: 25man raiding is made harder strategically (it often is anyway due to the nature of organising 25 players), this would be done at design stage.
B: 25man raiding rewards higher Ilvl loot
C: 10man zoning forces a reduction in Ilvl on 25man loot- no facerolling of 10man content for 25s
D: Separate the Achievements again, especially realm firsts, they are two different forms of competition, don't put them in the same boat
This would achieve 3 things -
Provide a greater challenge strategically for 25man raiders (the part of 25man raiding I used to relish).
Provide no incentive to drop to 10man from 25 (lower ilvl rewards wont help you that much in 25man and maintaining the combined lockout it would give 25man no real reason to raid 10 except cheese progression and hinder it later on in the tier)
No punishment for 10man guilds, they wont need the extra Ilvl to raid 25man and will still face a comparable challenge to 25man
As many others have stated I think the 15/20man idea is the "worst of both worlds" solution.