Well, I'm not sure if it's a good change yet. It sounds interesting at the very least, though I fear that the Engineer is being simplified. I sort of liked that he was a bit more complicated than some of the other professions.
See, I don't see this as simplification... at least not in a bad way.
In the previous build, Mines simply did damage, and were a little more difficult to use in the heat of battle (had to switch to the kit before you could start laying them down). You could lay down up to 5 mines, detonate them one at a time, or detonate them all at once... and that was all you could do with the skill.
Now, mines have a knockback effect, remove boons from enemies, and they can be used instantly. You can either drop a single mine at a target location, or drop a minefield (I would guess this contains 5 mines) directly at your feet. On the flip side, you lose the ability to individually place and detonate multiple mines. Certainly the use case of the ability has changed. Instead of being used for area denial, but requiring a decent amount of setup, mines now have more utility, both in chasing and escaping enemies, and are quicker to use. My guess is they determined that groups of engineers were coordinating to lay down huge minefields that were just too good at area denial when used in that way, but no one was using them in direct combat situations, and they wanted to remedy that. (Edit: Actually, reading up on mines, it looks like people were just plain not using them at all because they were considered bad).
Now if the change is ultimately for the better is going to depend on the numbers. Do the mines still do the same amount of damage (and does the minefield do around as much as 5 mines)? What is the cooldown situation with the ability, and do the placed mine and the minefield share a cooldown (and for that matter can you have both out at once)? Also, what happens to the trait that currently removes boons from enemies hit by mines, if mines now do that automatically.
Until we know the numbers it will be impossible to tell if the changes are truly better (I'd guess yes), but its certainly not dumbed down. And it's not as if it's suddenly less fitting of an ability for the Engineer. Not every utility skill needs to replace your current weapon, especially when it only had two abilities to begin with (place and detonate).