They probably did it because simply shortening the detonation time could've potentially made it too powerful, so instead, they repurposed it entirely.
Personally, I never got any good use out of Ink Mines in the first game, except by leaving one on the tower in Tower Control, but now, I actually think they're pretty decent. They're nearly inescapable, so you can place one in any choke point to automatically alert your team to when someone is trying to make a push. I don't think I'm all that good with them myself, but I've encountered some very good Ink Mine users online, and they're scary. Multiple Ink Mine users can be even more troublesome. I was skeptical of the Ink Mine changes at first, but now I think it's actually one of the more effective changes they've made to weapons in this game.
As for the comparison to Point Sensors, I like to think of it like this: Point Sensors are more active, while Ink Mines are more passive. To make good use of a Point Sensor, you have to have already seen your target once, however it also takes less time to properly use. With Ink Mines, it takes more effort to find the exact spots you want to place them, but if you put them in good spots, you can tag enemies that you never would've seen coming otherwise, which can't be done with Point Sensors.