I don't know of anyone who at the end of their first summer on a 70% contract was not either offered 100% straight away or 100% starting the following spring. So I don't think it's a reduction in pay for a few years, that's not accurate at all.
There's no doubt there's a lot of things wrong with jet2, and a solid rostering agreement and significant improvement to terms and conditions would go a long way to making it more of a career airline. However, ultimately it's a business and it would be negligent to its shareholders if it didn't try and minimise its costs in all areas, including pilot recruitment and pay. I don't like it, but I can understand it.
Ultimately, jet2 isn't the best place to work, but it's far from the worst. It's all about what you want. If a UK base is important, it would be foolish to dismiss them just because you might spend a few months on a reduced salary.