It's time for the series to be shelved for a bit. Let the fandom percolate, let the sales increase over a generation.

We've seen bigger arguably better FPSs come and go. I don't get how many iterations of Metro have been made when the likes of Red Faction, Turok, and Timesplitters have zero publisher interest.

If there is a 3rd Metro book for the game to be based off of, then sure, I don't see why they can't make a sequal.

Nobody buys PC retail games anymore.  Metro 2033 is estimated to have sold over 3.2 million copies on Steam.

Steam Top 100 Estimates from April 2014

They gave away the game for free though through their official Facebook fanpage.
http://www.eurogamer.net/articles/2012-12-11-thq-gives-away-metro-2033-on-pc-for-free

That's how I got my PC version and considering the Facebook page has over 1m likes, many others did so as well.

Also the THQ Humble bundle, which sold 885k copies and barely contributed to the revenue of the game. (same as selling ~15k copies at full price)

As Barozi sais, how much of those sales convert into good revenue for them. Steam can have great sales but 1 million copies sold on a cosole are usually way more profitable tha 1 millio on steam. PC users can be very scabby when it comes to waiting for sales. 

Didn't the developer say the Metro Last light was more profitable then metro 2033 by its first month?

I got metro 2033 from psn plus and on PC through the THQ humble bundle. Have never played it though. Thinking about the redux when they get a bit cheaper for my ps4. Does it warrant the upgrade?

You got last light on plus not 2033 which is the prequel. 2033 isn't on PS3 at all. Get redux because the series is fantastic and they improved 2033 only in requx. Getting all the trophies for them right now. 

On right. I always get them confused. Thanks!

2033 sold a million copies on PC alone http://www.ausgamers.com/features/read/3232525 it was quite the sleeper hit, that was before the giveaways and the humble bundle. Last Light first week sales were 3X the first three months of the first game http://www.polygon.com/2013/5/31/4384350/metro-last-light-sales-success. Add in Redux and I would say the series has done very well for it's self.

4A have moved countries (since the Ukraine went to shit) and have been expanding. They are working on a new IP and also possibly another Metro game.