Shalom R' David,

It's great to hear from you!

I certainly understand the logic of your question. If a wall is slanted, it would seem to be an unstable surface; after all, the Gemara notes that water would slide right off it. How, then, could it serve as a functional place for people to rest their heavy burdens?

To me, the most straightforward explanation is that we are dealing with a gentle slope rather than a steep one. Just as the Gemara explains that the slope is moderate enough that a Sefer would stay put without falling off, the same applies to the pedestrians in the public thoroughfare who need the wall for support; because the incline is gradual, it is convenient to lean against it or rest their packs upon it momentarily to re-adjust their load as they travel.

The point is that since it is functional and frequently used by the public for this purpose, it retains the status of a Reshus ha'Rabim, unlike a typical wall.

May you continue to attain greatness in Torah and Yir'as Shamayim!

Bi'Verachah,

Yishai Rasowsky