In any case, I think some posts are missing a point about the executive producer role: it isn't really a job, it's a position in the power structure.
For Steve Moffat, it means that there's nobody on the actual production team who can overrule him on creative or editorial matters apart from the other executive producers (all anyone else can do, ultimately, is mutter 'Do you think that's wise, sir?' or 'Well you can take my name off the bloody script then!!!'), and they are there as representatives of the BBC Wales hierarchy or to handle the mechanics of production.
If Steve's workload does play any role in the decisions over production schedules , then it's his workload as a writer, and in exercising his right to check the results and make changes if he's not happy with it, rather than just saying 'There's the script, look forward to seeing it, hope it comes out well, I'm back to my typewriter'. How does anybody else take that over (apart, obviously, from him writing fewer episodes... as RTD did from season 2. Hmm, SM has done 6+Xmas, 5+Xmas, and 4+Xmas, IIRC).
Decisions on the production schedule, number of episodes, etc, are made at least one step further up in the hierarchy... if Steve's exec producer trump card is a king, the people there have got aces. In that sense, one problem might be the job roles of the other exec producers: Julie Gardner was head of drama at BBC Wales, Mal Young was head of drama overall, whereas I don't think their replacements are as senior (the involvement of someone senior is both reassuring and worrying - it can mean that a project is important, or that it's seen as risky and in need of close supervision). Who no longer has anyone with an ace actively working on it - which could mean that it's no longer important, or just that they think it can be left to take care of itself.