The only thing that is dropped to 0 is the block subsidy, and will happen in 120 years. 51% attack has nothing to do with it, and will always be possible to accomplish (although, unlikely) as long as there's the Bitcoin network.
This is what I was wondering about, In past replies, some were stating miners will only earn through transaction fee commissions, and I think there was hardly any reply focusing on dropping the block reward in btc fractions.
If the case of bitcoin reaching above 120 years occus then bitcoin should have at least 20Trillion marketcap, which is a very good possibility and I think can be achieved.