I think that’s an easy one to forget about. ..... It’s much easier to keep recruits around when they’re actually getting to play. :)
You do realise that's the exact reason that guilds who are Mythic quality will be looking at mostly post-expansion shifts, yes?
It's not realistic to hit 20 before the expansion. Some guilds will likely be able to pull off bulking up to about 15 or so especially if they're 14/14H because at that point there'll be people more than willing to sit and so on. For everyone else, they'll want to finish content, not run around.
Which essentially ensures that barring rare exceptions, every Mythic-capable guild will be not-Mythic-ready for Warlords, unless there's some massive leverage to draw on. And that's getting past the volatility of the 10-man guild losing a few players to the expansion. Even if their roster was completely in tact, it's not just 10 more people needed, it's somewhere around 12, and if you include expansion end chaos, that jumps up to 15 very fast. "You only need 10" is one of the dumbest statements possible to make about this situation.
Tack on the concept of ensuring said recruits are of good quality, and many will be spending the entirety of T17 and possiblity some of T18 simply getting to the point of being Mythic ready - beyond the gimmes. That's not a small affair - it's not 6 months and done. The 6 months until the expansion is very limited on what it can actually achieve, and the more time a guild is spending during current content getting ready, the more bored people are by easy content*.
While 10-player guilds who .... higher pool of potential recruits.
Well, not really because of the previous point you already made about expansions being volatile. Any players who do leave won't need to be shed, and given the sorts of experiences as a 25H guild, that's almost close enough to where a 25M will naturally transition to 20M (occasional 1-2 recruits required).
Ultimately, we feel that the long-term benefits for everyone .... even for players in guilds facing this transition.
It'd be great if you could elaborate on this beyond the normal stuff that's been on all the PR announcements
As it is, you're at a point where 10-man and 25-man raid environments are more or less having decent content. Sure, one boss swings to be harder on one mode and easier on another mode, but by and large the variations are tiny - and they swing both ways. The actual instances you've had encounters that had huge holes (cf: Norushen, Shek'zeer, etc.), it's been more of a case of something very simple being missed by whoever was in charge of that (it really doesn't take much math to plot why Norushen wasn't a good trade) or a lack of willingness to use creativity in spacial environments (e.g. superelectrifying the outer 40% of Lei Shen's platform, or chunking Al'Akir's platform in half or shrinking it or one of several other solutions in 10-man).
And when it comes to encounter creativity/variety, I hope it's not all about the couple of examples used like Spellsteal and Enslave. Even back from BC, those were rare, not actually all THAT special (in fact, if it weren't for the specific Lock spec, ranged tanking would have been done a lot more by Shadow Priests and Boomkins and so on), and mostly gimmicks that get old fast that reintroduce a failure point for raids even starting (Mage unavailable? Screwed. Recruiting 2 of each class would instantly get to an over 30 roster for a 20-man group. Some groups simply will not see people of specific classes - as a 25-man we only ever had 1 DK, sometimes we had no Locks, etc., simply because those classes just happened to have low availability of quality people).
Encounter creativity should mean things like the Sons phase, Inner Demons, Lei Shen's platforms, Norushen's trials, good ways of employing council fights like Omnotron as opposed to Spirit Kings when RNG existed, etc. I would say it include things you've been employing for Healer mechanics like the Shield mechanics or Baleroc, but I think most of those have been terrible in a way that 10-man and 25-man had nothing to do with. I don't see where we're worse off now or limited to any unacceptable degree that would make the benefit of introducing this much chaos worth it - so it'd be great to hear why you think it is.
I also don't see why you feel you're THAT limited to where 15-man would be unacceptable instead of 20-man. 15-man still leaves the door open to expect a decent roster with one of every class as long as you don't do something silly like leave an ability usable only by one spec or make things impossible without that spec.
Either way, I hope you don't shift THIS 2 expansions down again.
*None of this has gotten into the destruction of a close-knit guild environment which you specifically wanted to recognise as a good thing and shored up. Or the cynical idea of $$$ on server transfers.