Well the only way to make it less possible to fall off in sideview mode is by putting a wall behind it unfortunatly.
I really feel they completely forgot some things when it comes ot that camera mode.. but oh well.
However, what I do want to point out is that there have been plenty of Sonic games that had 3D levels or entire games. The best of all of them is Sonic Generations which lets you play each level in 2D or 3D mode.