Yeah it's potentially really good value, especially as the 10 year period doesn't 'activate' until we get promoted to the Premier League. If that never happens you'd never need to buy one again, if you're young enough it could last 30, 40 years. The downside is that the really loyal fans have already purchased a 5 year season ticket which was made available 2 or 3 years ago, so they miss out.
The risk would be trusting that any future owner will continue to honour it if the club changes hands, which seems very likely to happen in the next 10 years.