under current terms, the repayments are in effect the same as if there was a "medium" tax band which applies only to graduates, but which you can opt out of with a big enough lump-sum.
without the student loans, income tax is zero for the first £12k ish, 20% on the money earned between £12k and £50k, and 40% on what is earned over £50k.
with the student debt, its exactly the same as if there was a tax bracket of 29% of what is earned between £25k and £50k. The only reason it is done as a loan rather than a tax is so that the debts of those who leave the UK can still be pursued.
You are right that the government can change the thresholds and terms, but in general if the degree will give your DD a higher earning potential, it will be worth it, and if she doesn't end up with a high-paying career she won't pay back much anyway. if she doesn't do a degree what will she be doing/earning in 10 years time? what doors will the degree open up?
the loans system is set up so that those who benefit hugely from their degree and secure a job with £200kpa salaries help to subsidise those whose careers require a degree but whose repayments will never meet the cost of their education, eg nurses and social workers. if you want to avoid that you can always pay the loan off early.