"then how come Christ speaks of lost sheep?"
His sheep were lost until He came. That's the point of His coming. When He gathers the last that are His, He will come again as King and Judge, but in the meantime, all that He has gathered will not be lost again or taken from Him.
Your theory does not accords with "there shall be joy before the angels of God upon one sinner doing penance" that follows the discourse of lost sheep in Luke 15, and precedes the parable of the Prodigal Son. The recovery of the sheep is through penance. The Prodigal son parable elaborates on that.