How much better could they have been with proper training?
I absolutely agree that good training is desirable and likely to produce better teachers. The problem for me, however, is the number of teachers who feel that their PGCE was not just a waste of time, but positively detrimental to their teaching - that they became much better teachers when they realised that much of what they had been taught on their training course was wrong.
There's been a lot of threads on Twitter making this very point over the last week, with teachers talking about their experiences of the PGCE. Some courses are reportedly excellent, but others are reported as being even a few years ago still stuck in the past, ignoring or dismissive of cognitive science and related evidence-based approaches. With something like 50 different teacher training institutions, the quality still appears to be variable, a point made by the Carter Review of teacher training two years ago.
The Carter Review also pointed out specific areas of weakness, particularly in providing effective training on behaviour management. This is also something that comes up over and over again in blogs and on social media, including having been taught on their PGCE that the main cause of disruptive behaviour is boredom, and that teachers are therefore to blame for it because their lessons are not sufficiently 'engaging'.
Ideas like this, along with one-sided promotion of 'constructivist' teaching methods, had a stranglehold on much teacher training for a long time, and IMO have done a lot of damage to education in this country. We really have no idea to what extent that's changed, so it's inevitable that there will still be scepticism about its value.