Getting a working qc to reasonable scale is the hard part. Once you have done that most of the hard engineering problems are solved. I doubt doubling its size at that point would be very much of a problem.
If we are making (uninformed) predictions, i bet we wont see QC solving 1024 bit RSA in the next 15 years (at least), but once it does it will only take a year or two more to solve 4096.
It will likely work for a while, but it’s a fundamentally wrong approach and you’re going to be exposed into recording & decryption attacks, instead of breaking your encryption today, I just store all your communications and wait for the next qc to be online, then fish for stuff that is still useful.
It’s a silly approach if the timeframe is 50 years because most secret information goes stale quicker, but if you’re just waiting for say a year…