Never mind how much more poorly it will run that 5th version of iOS than it runs the current version of iOS... I own plenty of old iPhones, some of which I have updated to the latest supported version, and others not. It's been my experience that there comes a point where upgrading feels more like a liability.
The form-factor of the SE alone is effectively 5 years out-of-date. There's no gesture-based navigation, it has large bezels, relatively low pixel density LCD screen, nearly 1/4 of the charging speed through a slower data port. The iPhone SE is a perfectly fine phone, but it really is a budget-phone experience at a non-budget price. Now, the fluidity if iOS itself carries the experience quite a bit. No question that it's a level above Android in all facets of user interaction and overall polish. However, Android is perfectly fine, and the better picture quality (of the display) of the 6A will contribute more to the user experience. The so-called benefits of the Apple ecosystem is also vastly overstated, and pretty much anyone who only uses Apple devices could switch to an Android phone and adapt with little friction. I'm pretty sure that the majority of iPhone users use a Windows computer anyways.
The reality is that the SE is just not competitive for what else you can get at the same tier. Personally, I would rather be on the look out for a good deal in the near future on an imported Nothing 1 phone than the 6A though.