In my fantasy world, the original Circus campaign would have lasted around a year with a typical singles run, then they'd re-release the album (as was all the rage back then) around its first anniversary in November/December 2009 with Amnesia as the lead single - it's got a bit of a Christmassy feel so is absolutely perfect for that time of year. (3 could have gone on that re-release too, and been the second single from it.)
Anyway, Amnesia blows up, gets played everywhere you go and storms the charts worldwide just like Womanizer did a year earlier, the album gets a strong second wind (especially with a follow-up like 3), and all is great in the world.
I strongly agree, though Amnesia or no Amnesia, it would have been way smarter to re-release Circus than do The Singles Collection.