re: Tell us what those reason are, Nick
You may be right.
But the argument that a 2-3 km depth for "habitability" also has its merits and is more widely known than your own deeper level claims.
At least from appearances, there is a 1-3 km "floor" to the scabbing of the surface by what appear to have been fluid or glacial processes. Impacts penetrate that floor, but very little that eroded the surface did -- except in Valles Marineris, where conditions were apparently different.
Is/was the top of the permafrost layer 1-3 km below the general surface? How deep is the permafrost layer? Is it global?
What is below that?