What if there's tens of thousands, or hundreds of thousands of new transactions per block
This is not possible unless there is a fork and we change the block weight or block size.
Right now, if too many transactions are sent, then the excess transactions that don't fit into the next block must wait until a later block. Those that want fast confirmation of their transaction therefore increase their transaction fee to get into the next block. The transactions with lower fees are then left waiting a VERY long time to confirm. Eventually, people that aren't willing to wait abandon the Bitcoin blockchain and use some other method for making their transactions. Those other methods may or may not Bitcoin off-chain transactions (such as Lightning Network or Account based services like Coinbase)
All those transactions in one block of 1MB?
No. They just wait for later blocks.
My understanding was that you needed the full blockchain for maximum integrity.
You need to have
received and
processed the entire blockchain if you want to be 100% certain that you have not been fed any false information about the current state of the UTXO. However, you do not need to have stored/kept that entire history. You only need to keep the UTXO.
What if everyone pruned their blockchain, would that mean there wouldn't be any full copies of the blockchain?! Wouldn't that be a problem?
Yes. That would be a problem. Fortunately, there are many of us that WANT and are willing to store the entire blockchain for various reasons. If you are able to store the entire blockchain and you want the entire blockchain (or you just want to be able to help other new users by supplying the entire blockchain), then you are welcome to do so and we appreciate your willingness to do so. If you are unable to , or you do not want to, then we can manage without you.