Its about how long it takes to become a solicitor or barrister or judge. So now its down to university's having the space and courses. Schools pushing kids to become legal people.
This assumes that solicitors and barristers never change course, therefore the Criminal Bar and criminal solicitors' firms will never recruit from people working in other fields. That isn't so - plenty of people who go into the legal professions do change into different fields for various differing reasons. It's not too much of a stretch to work out that people who moved from legal aid crime to exclusively privately paid crime work, for instance, would go back to it if pay rates became more realistic; ditto people who moved away into other fields of law. Then there are the people who always wanted to do crime but didn't because the pay rates are so awful. And people who would just like a change.
It also assumes that only solicitors and barristers can represent people, which again isn't so. Legal executives have rights of audience in magistrates' courts, and can gain higher rights relatively quickly.
As for judges, you don't have to have been practising in criminal law to sit as a judge in a criminal case, so again we don't have to wait for a whole generation to get their A levels and work their way up to become judges. If Sunak can find 15 Rwanda judges as quickly as he claims it shows it can be done.
I'm not being starry-eyed, I know that this won't happen overnight and I'm quite sure Starmer knows that. But none of that means it cannot happen at all.