Too bad they can't do it on the exact 40th, but that pushes it off of Friday, so that was a no-go.
Blue Bloods was probably indirectly keeping Five-Zero around for a few extra years. It attracted the same demographic -- older viewers. Blue Bloods actually has the oldest average audience on network TV (I heard 70!), but Five-Zero is up there, too.
That's why the series had a lot of patriotic and pro-military undertones. The older crowd doesn't want to watch a protest piece about America being bad and its military being evil (and to be honest, I don't want to see that, either).
This is also why Five-Zero didn't push the envelope with any of its main characters doing anything particularly edgy or controversial. For example, notice none of them had any kinds of gay flings or romances (well, unless you count McGarrett and Danno!) Again, they don't want to upset the old people.
I also fear that the finale will be mediocre. I'm afraid it will be too focused upon wrapping things up with everyone and everything, and not enough focused on actually sending off the series with a great final episode.
Lenkov's "I've had the idea for this for a long time" line scares me. Reminds me too much of a short-lived (one season) series I really enjoyed called "Life On Mars" (the US version). It was a fascinating series about a 2008 NY cop who woke up in 1973, and the series mixed in regular police procedural stuff with a sci-fi element, where the main character kept getting little clues as to why he was in in 1973 and how to get back "home" to his own time. Then the series went on hiatus and lost its momentum, so they had to hastily put together a series finale. It ended up terrible, and basically took a really lame "it was all just a dream" explanation for the whole thing. Lame!! I was so disappointed. In that series, the showrunner also claimed that this was his idea for resolution all along. I guess I'm glad they only took one season to get to that awfulness!
I guess we will see.