Instead of differentiating their phones based on features, they should simply differentiate them based on screen size. That would allow them to meet several price points without making their customers feel like they’re cheap.
Remember, the SE was released outside of the usual iPhone release cycle in March 2016, so I think intent for a longer cycle was clear.
Lots of people predicted between 2 and 3 years before the SE got an update, so if March 2019 passes without an announcement, then who knows what's next.
Also, I think there's a high probability they're trying to work out the kinks in an SE-sized phone that is all screen. I'd be surprised if they released the SE2 and it has a home button.