Blackjack the Titan
It’s been a while
To be fair, rollerblades are way easier to animate in 3D in comparison to a bike. Not saying the bike is replaced, but I wouldn't be surprised if it were. And it the terrain really shouldn't matter, because it's not like these rollerblades act like real rollerblades. If the designers need rollerblades to go up a mountain, they will do it.
You're right, screw physics, the bike in previous games went UP stairs, so rollerblades could be possible.