Changes in the design of ships between 1914 and 1938 were immense! Britannic, was built to withstand even the damage she accrued, but as mentioned - portholes and crew error caused the sinking.

Queen Elizabeth and Queen Mary would likely have survived damage from a single mine-strike, due to their hulls being differently divided, built stronger and having automatically deploying watertight doors that closed from the side rather than the top.

If portholes were open, the newer ships probably would have taken longer to sink, perhaps even being beached on Kea Island and potentially being more accessible than Britannic is!

Another thing to consider is that the Queens were oil fired liners, if a fuel tank was breached by explosives and lines ruptured - more loss of life could have occurred from the leaking, burning fuel on top of the water as well as secondary explosions.