Meant to comment way earlier on this, but didnt have chance to.
A few examples of games that started as 32 bit that successfully made the jump to 64 bit compatibility
Supreme Commander
Eve Online
X3 Terran Conflict.
Plus the examples posted above like Half life 2
It can be done. It already has been done. X3TC, and SupCom are still 32 bit games, but have been updated to utilize more than 2 GB of ram, and multicore support.
If the new Sins update adds these features PLUS uses some elements from The Optimization Project, or creates an optimization scheme of their own then this "Mature" engine will most certainly have an extended lifespan for at least one more add on. Plus opening up more options for the mod community. Therefore leading up to the next incarnation of the game in a year or two's time.
If there is to be a Sins 2 think of Sword of the Stars. The new SOTS 2 engine is light years ahead of the original. That is the kind of progress i would expect from Sins 2, and that will take lots of TIME to do. I would also expect to see a completion of the Sins lore, and the revelation of the Vasari's "Unknown Enemy". Perhaps Sins 2 will be the three known factions allied to fight the Unknown Enemy. We wont know until Ironclad sheds some light on this.
In the meanwhile one more expansion will keep the interest of the current fans, and introduce Sins to more new players. That is why there should be at least one more swan song for the Iron 1 engine. The fact that the dev's want to put time, and effort into this shows that there is indeed still an interest, and a viable market for Sins. So i say do it. Then work on the 64 bit Iron 2 engine (call it "Steel") So IC can shock and awe us all, and put SOTS 2 to shame.