As much as I detest P2F, and even paid TRs, I don't blame the wannabes who sign up for them.
No one can predict what the recruitment market will be like when they finish training. Ask anyone who started in mid 2007 (when recruitment was booming) and finished early 2009.
With the exception of sponsored cadets (when was the last time there was one of those schemes in EU?), we all took a risk with our flight training. Some of us are lucky and emerge into recruitment boom, some aren't and emerge into a period of stagnation.
If you finish your training and the only jobs available are P2F, what do you do? You could take the moral high ground and say "no, I refuse to pay to work" - but what will that achieve? There will ALWAYS be someone else willing to take the job. If you can afford to hold out for something better then you may be lucky. But you'll be sitting there watching you £70k license getting staler by the day while the flight schools turn out hundreds of fresh competitors every month. In that situation, what would you do? If it were your brother/sister, what would you advise them? Would you tell them to hold out, or would you tell them go for the P2F, suck it up for a few years and hopefully move on to a decent job at the first opportunity? I honestly don't know what I'd do.
So what's my point? - Wannabes will NEVER be able to stop P2F schemes and the degradation of T&Cs that they cause.
Management obviously have no interest in stopping such schemes. There is one group of people who are both responsible for the inception of P2F, and this same group are the only ones who are (may be) in a position to put an end to them. These people are of course, current unionised pilots.
Iberia (who owns Vueling if I'm not mistaken) have one of the strongest unions in Europe. If the Iberia union (with the support of some Vueling pilots) went to Iberia management and told them they were going to down tools until P2F was scrapped, how long would the scheme last?
I can't imagine P2F is a core part of Vueling's business model - it obviously improves the bottom line slightly, but would management really be up for industrial action to ensure the scheme continued? I doubt it.
This same principle can be applied to both the BMI and Easyjet P2F schemes (I don't know if these schemes are still operating), both airlines with good union recognition.
In summary, if you don't like what P2F is doing to the industry, don't blame the wannabes that are signing up for it. Blame the pilots/unions who allowed their management make P2F the only option for the wannabes.