I'm sure everyone has given their opinion on this so I'm not going to bother with mine just now. Instead, I'd like to focus on a scapegoat that's not really a scapegoat. SGU was canceled for the same reason as Atlantis, and SG-1: too few people watched it to justify the cost.
It's easy to blame online piracy, but it's not that simple.
There's no easy way to measure how illegal downloads affect TV shows because how decentralized online theft is. But we can still make some pretty good guesses, so long as a few caveats are taken into account.
There are more ways to pirate TV shows than just BitTorrent, but data for those methods is even harder to come by, so I'll exclude them. Anything number I give is going to have a pretty significant margin of error, but that's true for numbers you'll get from anyone, especially since I'm not aware of anyone that measures every possible source.
Not all shows have the same online following. One of my sources for the most downloaded TV shows in 2009 shows the number of downloads for Heroes (#1) exceeding the average number of weekly total viewers, while Grey's Anatomy (9th) was only downloaded once for every 8.9 viewers. Dollhouse, while it was on, was one of the most DVR'ed shows despite having a small enough audience to get nuked. It probably had a larger relative online following that a show like Grey's did.
That bit needs its own caveat: I don't know if the downloads listed for each show from one source is for the entire season, or also is an average for a single episode like the traditional viewers.
One source reported that the series finale of Lost scored the all-time record for most downloaded episode, with 900,000 downloads in 20 hours and would probably reach 4-5 million when all was said and done (without counting forever). That's about 37% of the number of the people who watched the finale on television but still less than one site says Heroes was getting on a weekly basis.
If that's true and accurate, then it's doubtful that Heroes actually had more people downloading each episode than were watching it on TV. The figure of 6.5 million downloads against 5.9 million viewers (per episode average) probably breaks down to roughly 300,000 downloads per episode. Not only does that make sense compared to the Lost finale, it gives us a ballpark perspective about just how many people are opting for piracy over television.
It's not clear how many of those people -- for Lost or Heroes -- were outside the country and wouldn't have affected U.S. ratings even if they had watched the shows on TV. If we assume all of them were Americans, which is silly but I don't see what other choice there is, the number of people who downloaded Heroes every week wouldn't have moved the TV rating by even a single point.
One point in 2009 is less than it is today, but today a single household rating point equals 1.1 million homes (not people). The numbers get a little convoluted after that and there's no way to derive the number of viewers from the rating, but if you assume that 1.1 million homes equals 1.1 million viewers (in reality it's probably closer to 1.0-1.2 viewers I think) then you could shift all the people who pirated Heroes to their televisions and it would only have gained 0.3 in the ratings.
For a show that already has about six million viewers and a rating between 8 and 9 for the finale season, a 9.3 rating instead of 9, or a 8.3 rating instead of 8, isn't going to magically save it from the chopping block.
To illustrate how hard it is to come by solid information, one source I have for the top 10 downloaded shows in 2009 claims that HBO's True Blood averaged 12.4 million TV viewers that year. In reality, it was closer to 4.2 million.
Without any other numbers to challenge the download figures, however, I have to accept them at face value. If 300,000 people were downloading Heroes each week and it was the most downloaded show on broadcast television, chances are that a cable show like Stargate Universe was closer to 10,000-50,000 downloads per episode. At that scale, shifting all pirates to television would mean gaining less than 0.1 in the household rating, when it probably needed a 0.5 increase just to say alive.
The numbers are dicey and there's lots of room for interpretation, but it's very unlikely that piracy has ever affected any show enough to mean the difference between cancellation and renewal. And that seems to be the case for Stargate Universe as well.
It's far more likely that the decision by SyFy to move SGU from Friday to Tuesday nights is what killed the show. The ratings had stabilized from episode 13 on through 20, averaging 1.421 million viewers across that span. The season two premier on a new night dropped to 1.175 million viewers which will probably end up being slightly higher than the average through the first half of the second season.
The only explanation for that is either the hiatus between seasons drove people away, or the scheduling change did. It was one or the other.
But my main point is that an additional 10,000 to 50,000 viewers wouldn't have made a difference. SGU needed 1.5 million and up, and even the record number of people pirating Heroes in 2009 couldn't have made up that gap.