[Marcello]

Some considerations:
Cluster bomblets are quite capable, for example the M77 used in the MRLS is rated as being able to penetrate four inches of RHA.
Most tanks, including many modern ones, have substatially less top armor than that, often as little as 40mm (protection level against fragments and such).
The Abrams seen from the top aspect is probably all plain RHA no more than 80mm thick at the most and probably less in several areas. Upper glacis appears to be a relatively thin steel RHA plate designed to cause ricochet and fuze failures against Sabot/HEAT but somewhat weak from the top, ditto for the turret roof (such armor scheme makes sense against the threats it was indended for).
Even the tanks where considerable attention has been paid to the top aspect, such as late Merkava, some late Leopards etc are still vulnerable in the engine deck (you still have to radiate away the heat somewhere).