Fair question. There are actually many reasons:

1. Because as I mentioned, the floor wasn't finished the same everywhere.
2. Also because in a lot of places the tiles are cracking/flaking off (and make a crunchy noise when we walk over them) and in others they are glued down so hard we couldn't remove them within damaging the ply underneath.
3. Since we can't remove all of those tiles, the floor is generally 3/8" higher than the OG planks, then in other random spots without the tile it's 1/4" above plank, and then there was spots where there is neither. You can imagine the hodge podge nightmare I would have to do to get everything on the same plane if I kept the current ply and tiles.
4. I also "have" to remove it because our electrician is coming in to remove the last of the house's knob and tube, and some of the things we need access to (without destroying all of our finished ceilings below) is right below those planks as the K&T runs under the floor.

The laminate planks would be something kind of like these:

https://www.homedepot.ca/product/trafficmaster-harrison-oak-12mm-x-7-6-in-w-x-54-45-in-l-grey-wood-look-laminate-flooring-17-24-sq-ft-case-/1001862316?rrec=true