Since Siri AI will need 12 GB of RAM to do everything "on-device", it makes sense that the base iPhone gets a RAM update too.
It would be embarrassing for Apple to introduce new iPhones in March 2027, none of which would be able to run the new Siri AI properly.
Apple is late to the AI game; everything depends on getting the feature out to as many people as possible.
My tip: 12 GB for Base and Air2, 9GB for 18e.
Base will be more expensive, but you can always get the 18e if you need cheap.