Firstly, it is foolish to say that the line training that is offered to those that pay is to a lower standard. The CAA (or any other regulator) has no remit to get involved in abolishing or restricting the pay to train ethos that is being adopted nowadays. As long as the training standards are being upheld, then that is all that the regulator is responsible for.
The fact that those who opt to go down this route ending up as 'contractors' and not employees, does bring a degree of comfort to those employees and especially those who are well entrenched as employees with a period of service behind them. If you read a number of other forums, it is clear that a number of pilots genuinely do not feel that the 'pay to train' endemic is a direct threat to their own terms and conditions, especially when you are a Captain.
Many will still see this issue as a 'potential' or 'unseen' threat and that is why issues like rostering, annual leave or whatever, are seen as more pressing matters and this is what the unions will be getting approached with.
If crews start to face pay to train practices that affect them directly, such as paying for command courses and LPCs etc, then unions will get a earful and employees will stand up and fight. Of course by that time the rot will have set in and it will be too late.
In short, low cost airlines have been fantastic for the consumer and a disaster for pilots.