The 12 month pyramid is a good idea. I struggle with deciding on when old sends are no longer meaningful . Of course, by using a 12 month pyramid it basically forces you to stick to short-term projects to maintain a pretty strong base. That may be good for you and I, but maybe not for more elite climbers?
Yeah the 12 months timeframe does favor project that doesn't take months or years. But it's an arbitrary timeframe, so you can pick a longer one if you want to focus on long term projects. I've been considering extending my timeframe to 24 months, not because I have some mega project, but because I just don't get out and climb that much.
Also for full disclosure, I certainly have not tried a 5.12d yet, not even close.
In terms of the original question, I think there's something to be said for breaking your established patterns. I feel like some people really like the feeling of having a big project to train for and devote lots of energy to, both for productive reasons (wanting a real challenge, the motivation created by an ambitious route, etc) and unproductive reasons (if your project is sufficiently ambitious sometimes there's no pressure to actually send, yet you can still delude yourself into thinking you're stronger than you actually are just because you did most of the moves on a 5.xx). Conversely, there can be productive reasons for choosing multiple small, easier projects (setting realistic goals, building the pyramid, taking the long view of progression) as well as unproductive ones (fear of failure, playing it safe). I think there is a case to be made for varying your projecting style for the sake of getting outside your comfort zone. It can also be dictated by availability of routes. I live in Western North Carolina where there's very little sport climbing. I'm pretty sure I've climbed all the sport routes 12b and below in my area, so when I climb at a local sport crag I'm limited to hard projects or repeating routes I've already sent. When I travel (which is rare) I hesitate to spend the weekend working a hard route and generally choose more attainable routes.
...if your project is sufficiently ambitious sometimes there's no pressure to actually send, yet you can still delude yourself into thinking you're stronger than you actually are just because you did most of the moves on a 5.xx).
I see this a lot and its one of the reasons I discourage big projects. I think consciously or sub-consciously a lot of climbers pick these types of projects so they can avoid facing pressure. That and I think some people think its better to be seen on something "hard", even if they're failing on it, than be seen on something "easy".
I tend to take things too far the other way, where I hate to be seen failing on stuff. I'm less inclined to challenge myself if there's a big crowd present, which surely holds me back at times. Ironically I seem to actually climb better in front of a crowd, but that doesn't mean I enjoy it
I also like to look in control in front of a crowd, especially if I'm climbing somewhere where most people are better than me (Rifle, Welcome Springs). I don't want people to think I'm struggling on 5.XX (generally their warm-ups) so I try and act like it's not a big deal, which almost always backfires and creates a vicious cycle. By not going hard enough, I end up failing anyway. It's definitely something I'm working on.