3 years is a reasonable expectation for decent support, and since Apple is still selling the Watch 3, it's fair to say that the SE, which is far more capable, will get that 3 years, perhaps longer.
I bought an SE in February of this year and expect to keep it until the S8 or perhaps the S9 comes out. I don't think that's unreasonable, and I don't think by the launch of the S9 I'll be seeing my SE show super slugging performance, unless I'm unlucky with the battery.
The SE does not have the EKG feature, quick charge, Always On Display, or Blood Oxygen (though the blood oxygen feature is pretty crap from what I understand).
I went with the SE (vs S6 at the time) because I wasn't sure that an Apple Watch was for me, but I suspect that at some point in the future when it's time for replacement and I'm contemplating the SE2 vs S9, I'd go for the S9. I'll be edging far closer to 50 by then and the additional health features will be more relevant at that time. All the same, I don't regret getting the SE, it feels like it was the right choice and I'm happier with it than I thought I would be.
