As I understood, in the block hash, the amount of zero's is the nonce, like this:
When there are more blocks found than in the average of 10 minutes, the nonce will increase. Then the hash is recomputed for each value until a hash containing the required number of zero bits is found.
But imagine the development of ASIC's accerlates expotentially, so the nonce for the required hash will be like this, for example:
I know there are more information required, but let's imagine this in 2020 if the development of new ASIC could accelerate.
So in theory there are only 28 (note the position of 4 in block hash) x 28 (position of e in block hash) = 784 remaining possible hashes. Would thatn't mean that the Bitcoin ecosystem with an insane difficulty will eventually runs out of possible hash combinations for blocks?