The bad news: no, there's no spiritual successor to the WRT54G. That's the only reason Linksys is still making a 10 year old product. People just love it because it's been by far the easiest to work with. Nothing else is going to be as easy to set up and get working.

The good news: there's a better selection of routers now that CAN use replacement firmwares.

The only downside of these newer routers is that, IMO, it's not the dead-simple process to upgrade. With the WRT54G, it was a simple matter of navigating to the administration tab, selecting the upgrade file and clicking a button. That was it. Now you have to choose between different versions of the same firmware, sometimes have to use the manufacturers' tools just to prepare the router for upgrading, and sometimes use preliminary versions of the firmware to prepare the router for the final copy. It's a bit of a mess, and IMO the communities around these projects are not great at simplifying the process for people.