Our progress in early expansions has always been behind - I honestly can't remember any time in any of the last 3 expansions where we had cleared the content on normal before the next tier came out. We were always pretty casual but at the end of the expansion was when we caught up. The exception would be in Firelands where we did do heroic stuff but I'm not sure what was different there...
Jump to Pandaria and it really was just like every other expansion. MSV and HoF - we were still doing those and we never even got into ToES before Throne of Thunder came. We couldn't get Lei Shen down on normal before Siege came out. We did have a number of issues with attendance and tanks during Throne of Thunder (as well as a server transfer) but it really is on par with how our guild has always done things.
I've wondered what we will do when 6.0 hits and we can't do heroic modes anymore because we don't have 20 people. Maybe we will have completed SoO by then - in which case people may be glad to have a break from raiding - but I know I would still like to raid. Maybe there will be another guild of 10 in the same boat and we just hook up together and do content as a PuG until the release of Warlords. I see a lot of guilds running 2 groups 10, or 25 mans in preparation for that time, so maybe I won't be able to find someone suitable. Or who knows, maybe we can PuG people to get to 20... now that would be an interesting and rather unsatisfying experience - unless of course I find a real gem that I'd love to recruit into Frostwolves.
Even when WoD comes out, do I think we will get to clear it on heroic and have to go to Mythic before the next lot comes? I am not sure, and I don't think we will - not being a pessimist, but realistic. If we do that would be brilliant! But I'm certainly not going to cry about it if we don't. And if it's anything like this expansion then we won't have to worry about Mythic content until we get to the final two tiers anyway. Which leaves me plenty of time for recruitment.
Looking at recruitment forums is rather miserable anyway. I see tons of people looking for guilds, and even more guilds looking for people. I'm hoping that some of our casual raiders who show promise can make the step to raiding - there are about 3-4 who look promising and I'll just have to see how it goes. Enjoy the present and our heroic raiding for now, and consider but don't be obsessed with the future, because time is short, as Robert Herrick's wrote:
Gather ye rosebuds while ye may,
Old Time is still a-flying;
And this same flower that smiles today
Tomorrow will be dying.