Worthwhile replacing battery of 1st generation iPhone SE?
Generally, Apple supports iPhones for seven years—so going from the date it was discontinued in 2018, would give a window of support until 2025. Sometimes, due to specific hardware being able to run later versions of iOS, it may be possible to go beyond that point, but is a gamble.
If you're looking at extending the value, the $100 cost looks like a good deal—as long as you don't need the latest-and-greatest. Keep in mind, though, that the proliferation of 5G networks and changing carrier support may factor into your decision.