The reason I don't think it will be a real issue (I do think it's worth considering, though, and I do believe it will have some effect on 25 man recruitment) is threefold. Unrelated to those reasons (which are coming up after the jump) I have to admit that this may vary by server: on Norgannon, if anything I'm seeing more new 25 man guilds recruiting and starting up the crawl through Gruul's and Magtheridon, so I may just be working from a glass half full through rose colored glasses state of ludicrous (and heavily over-metaphoric) optimism here.
- It will be impossible to tune 10 man raid content and 25 man raid content so that they are fundamentally the same experience. Imagine, for just one example, the Kael'thas Sunstrider fight in Tempest Keep. While it was possible to maintain some of the flavor of that fight for the five man and heroic five man version, the complexity and sophistication of the encounter was simply impossible to maintain when paring it down. A 10 man will have more variety in that regard than a regular five man, but it would still be impossible to allow an encounter to have the varied phases (Single Advisors, Weapons, All Advisors, Kael, Gravity Lapse Kael) in a 10 man encounter. It is not just a question of the encounters dealing more or less damage, it's a matter of how much an individual player can actually be expected to do at one time. 15 more players means a lot more complexity can be introduced, and 15 less players means that you have to reduce the 'load', so to speak, the encounter demands from each in turn.
- It is a simple fact: some folks raid for gear. People run heroics now, even though the encounters are more or less the same, for badges and improved gear. Furthermore, there doesn't seem to be any real difficulty in getting people to run Karazhan even though you could get better gear through badges from heroics because Karazhan itself also offers badges. I think if the difficulty of the 25 man raids is sufficiently higher and rewards better gear for completion of the more difficult content, that will be sufficient to keep folks interested in 25 man raiding. Allowing for these raids to be tuned for 10 man content won't mean that raids will avoid the 25 man versions, in my opinion. Especially when we have no idea what kind of boss tuning we'll be looking at.
- 25 man raids are not just 10 man raids with more folks tacked on. There's a difference in playstyle and more variety possible: 25 man raids allow for specialization. Enhancement shamans, retribution paladins, shadow priests and others add flexibility and raid-wide improvement to DPS, tanking and healing in a synergy that is impossible to achieve in a 10 man. As a result, I think not only will fights be tuned to a more challenging level in the 25 man versions of raids, but players will want to experience that content because it offers them a way to break out into what can often be derided as 'off-spec's'. A 10 man may not have the room to place a retribution paladin and a protection paladin together to help the tanking paladin's threat in the manner a 25 man can, for just one of many examples.