The interest rate isn't 9% for one, it is currently 3.2% you pay back 9% on salary over a certain amount.
The interest rate is only important to those that will pay it back in full before the 40 years are up.
Say you take out the £9500 per year tuition fee plus £5k a year minimum maintenance loan and you do a 3 year course. At the end of uni you owe ~£45k plus interest.
You graduate you do ok and get a fairly average job at £30k a year with average wage growth of 4% a year over the 40 years you are eligible to pay off your student loan you will pay back £45,000 and the rest would be wiped. The interest could have been 1% or 1000% and the amount you paid back would the same.
Say you do a bit better, you earn £35k a year with average wage growth of 4% over the 40 years you are eligible you pay back £75,000 back which is the same as 3% interest on the £45,000 you initially borrowed, the £30k remaining is wiped off. The interest rate of the loan isn't going to make a huge difference to the amount you repay and there is a good chance that you can make better use of the lump sum in savings or towards a property
All of this assumes no gaps in employment and earning above the £25k. I'm on one of the older student loan, I earned under the threshold the first year as it took a while to find a graduate job so I was waitressing in between. I had a short contract at one employer and decided I wanted to do a PhD. I then did a 5 year PhD. This was funded through a stipend so didn't have to pay back my student loan. I then started a post graduate job after finishing and did 8 years with them before being made redundant whilst pregnant last year. My sector has seen a loss of £2 billion of investment in the last year and job are scarce. I now am looking at 18 months of work and then will be looking at retraining and potentially taking a longer break because of the benefit me being at home has had on the family. I'm probably looking at part time work and stepping off the career ladder for a bit.
Aside from one of the loans the interest rate is capped at inflation so it's not necessarily helpful to think of it as a debt that is growing. The interest rate in terms of how much you are paying back is less important than the time you have to pay the loan back for. The plan 4 loans were RPI +3% over 30 years students paid back 56p in each £1 borrowed with ~1 in 4 paying back in full. The plan 5 loans are RPI again but paid back over 40 years and the students pay back 81p to the £1 with 1 in 2 students likely to pay back in full.
There are absolutely concerns about the cost of university that absolutely need to be considered but the repayment of the loan is not where I would be starting. Currently it is very difficult to get the full maintenance loan, the threshold for the reduction in maintenance loan starts at £25k household income, that is a single full time wage at minimum wage and hasn't been increased in 20 years. It is household not parental, if a parent with no income at all and moves in a partner as their child starts uni with an income above the threshold the loan amount gets tapered even though they would have no obligation to provide the uni child with financial support. The minimum maintenance loan in the UK is insufficient to cover the student accommodation costs in many halls of residence. Many student accommodation outside of halls are all year rents which then cause issues with costs over the summer holidays. The increase in the number of students needing to work to fund university is increasing so there are more students looking for jobs with less jobs available with the struggles in hospitality and retail industries are having.
The cost of living is having a real impact on students budgets during uni. There is a currently a £5k deficit between the minimum maintenance loan and the cost of living at university in many university cities. Students are leaving university not just with their student debts to be paid back based on earnings but actual debts too. Entering into a jobs market saturated by graduates with massive grade inflation it used to be that ~15% of students were getting 1st class degrees it's now 30-40% at some universities. Positions that were needing degrees now need high class degrees, jobs and courses that previously needed high class degrees now need Masters or PhDs. It used to be that you could get on to PhDs with a first class degree now you are looking at having to do a masters first so students are having to find another £10k to complete their studies plus living costs for 1-2 years whilst they get qualifications whilst they enter a jobs market saturated by very able and competent applicants. There's a lot of competition out there.
Going to university now means that you will be paying a 9% tax on your earnings for most of your working life. It absolutely needs careful consideration. It is a lot of money if you aren't able to make a benefit from the course. The "student loans" get a lot of the headlines especially the tuition fees, it is a lot of money but on many ways It is also not enough money particularly from a cost of living perspective and also from the perspective of what a parent is expected to contribute. It has never been a loan in the commonly understood use of the word loan. It is more akin to the schemes in other countries know as "graduate contribution scheme".
Well done if you got to the end!!