They Are Billions...but persistent?

Can anyone recommend a game like TAB but that doesn't end (unless you die)? I like the concept. But I'd like my game to continue in a persistent way rather than each time I fire it up having to start from scratch. Thanks.

It sorta depends on what aspects of the game you're enjoying. If you're in to the city building stuff there's plenty of great city builders out there, but if you enjoy the survival stuff it's sorta intrinsical to the game that you "survive" at some point.

The best fit I can think of is probably Frostpunk endless mode. It doesn't have the defense and combat stuff, but it still has survival elements. Although having that said the endless mode does eventually suffer from the problem of you basically having "survived" and there's little to no point in going on.

I'd also recommend looking into Oxygen not Included survival mode. It's a fairly different game, but it plays on a lot of the same concepts and is more of a long struggle rather than a couple of hours per level.

If you're more into the combat side of things I'd look towards the grand strategy genre, with games like Total War and Stellaris. Probably leaning towards Total War which is more combat focused.

Not much to really suggest unfortunately. I would suggest the Stronghold series and Evil Genius (with the great fan mod which balances the game, available if you serach).

Unfortunately there are really almost no games that fit your criteria. I have always been a defensive minded RTS player so I have liked the growth in that genre with games like Conan and They Are Billions but they are so high stress and limited possibilities.

I wish there was a genre that catered to something between a city builder and RTS more. Like TAB, but more focus on development and sustaining a compound/stronghold, more goals than just surive, and less constant pressure. I think Evil Genius might really be the best option for something like that.

EDIT: One i also should suggest that I haven't played yet is Rimworld. I think its similar in those ideas. I need to finally get into it soon.

If you don't mind replacing combat with surviving virus outbreaks, I think Banished is a good survival oriented city builder. More focused on building infrastructure and routes for your resources than a tech tree/upgrade path, though there are mods adding more progression to it. In the regular game you have access to everything from the start, it's mostly about figuring out a working construction order to make sure you get the necessary resources every season and managing your population.

@thebooknerd: if the parts of They Are Billions that you enjoy include creating barriers and defenses and trying to hold off a massive but persistent onslaught of enemies I would try Factorio in normal mode. Once you have a very large factory, an unending sea of mutant ants will continue to smash into your walls and test your defenses.

You will have to create a system that auto repairs your defenses and provides them with a steady supply of ammo. By the end you will be making tanks, mechs, drones and all kinds of high end weaponry to kill them at the source(s). The last time I checked my factory you can not play the game without hearing nearly a thousand turrents in a constant state of firing whilst consuming a river of ammunition courtesy of an effective weapons factory.