the developer will always upgrade the system following the growth of the technology.
That's not an issue. Bitcoin developers have already post quantum solutions.
But there are lots of 'shalecoins',
https://bitcointalk.org/index.php?topic=5134441.0 coins with no owner. With quantum computers, these coins will become active and change the Bitcoin ecosystem.
Satoshi had already thought of the quantum computers, and the possible decoding of the privatekeys if it became available, ..
His coins would be quantum secured, if he sent them to P2PKH addresses. But he did not and isn't doing.
and
privatekeys are somehow within the blockchain = quantum computer?
We will get the answer with quantum computers. As soon as possible.