The 105 will still defeat over 80% of the tanks in the world by hitting on the frontal armour, and will defeat 100% of the tanks in the world with flank or rear shots. It cannot defeat the frontal aspect of the Challenger II, the M1A1(HA), the M1A2, the Merkava, the T90 and most likely the Leclerc.
If you look at the potential threats, the 105, combined with the modern FCS as in the LeoC2, is still a very credible battlefield weapon.
Of course, the Leopard C2 can be upgunned to the 120, if the political will was there...........