The most challenging thing will be:

When the privatekeys of the lost coins with no owners - 'shalecoins' - will be 'fracked'.
The first group will then sell all these coins because they will then know that it is possible to get the privatekeys and that others could 'frack' them. No matter what the BTC price, they will sell them.

There are more than 1,000,000 'shalecoins' with a 10+ billion US$ value. So we can expect that there are private groups - not institutions like the link below - who want/try to build a supercomputer.

But thereafter 'shalecoin' https://bitcointalk.org/index.php?topic=5134441.0 will be solved and the blockchain will be clean.
That is important for the Bitcoin development.


"Quantum computers will soon outperform classical machines"
"The project is part of the EU’s €1 billion, 10-year Quantum Flagship initiative to kickstart a competitive European industry in quantum technologies."
http://www.engineersjournal.ie/2019/07/01/quantum-computers-will-soon-outperform-classical-machines/
https://medium.com/the-quantum-resistant-ledger/no-ibms-quantum-computer-won-t-break-bitcoin-but-we-should-be-prepared-for-one-that-can-cc3e178ebff0