Unless S4 is very poorly received (which I think is doubtful) there will probably be more Sherlock at some point in the future. There may be a longer break than usual, however. It's my impression that S3 and S4 were commissioned at the same time, but there isn't currently a commission for S5.
Looks like someone's bucking for more money to me. Steven Moffat has said he'll be surprised if they don't do series 5, he and Mark Gatiss have talked about planning the arc for series 4 & 5 at the same time while sitting on the catering bus during filming for series 3 and suddenly BC is naysaying. Doesn't take Sherlock Holmes to work out what's going on there!
Looks like someone's bucking for more money to me.
That's the least likely reason, IMO. They all love doing "Sherlock," and none of them need it to provide them with a ton of money. They're all doing very well with their other projects.
I think it will be the last. Everybody is way too famous and busy for this show, and even at the height of its popularity they weren't exactly getting to work about it. That said, will they make a special ten or twenty years from now? I don't see why not. I think it's likely there could even be a movie at some point.
"I said no camels, that's five camels, can't you count?"