errrr.... that's how indexing to RPI works..... The interest rate on the loan is RPI compounding every year and the RPI index effectively compounds every year as well. Inflation really does compound every year, just like the interest on a loan.
If the cost of my basket of weekly shopping is £100, it will go up by inflation every year. RPI is a (imperfect) proxy for inflation so the cost of my shopping goes up by RPI ever year. After 1 year, if RPI is 5%, my shopping costs £105, but after 2 years if RPI is still 5%, it goes up by 5% of £105 ie to £110.25.
Now suppose I didn't have £100 for my shopping in the 1st year so I borrowed the £100 at an interest rate of RPI. After 1 year, my loan has gone up to £105. After 2 years, my loan has gone up by 5% x £105, ie up to £110.25.
Can you see how they're both the same? That's what it means if the interest rate on your loan is the same as RPI flat. [The earlier loans were at a higher rate and so ballooned alarmingly rapidly.]
So I now owe £110.25 but it's really just like owing the same basket of weekly shopping that I started with. In "value" terms, my loan is the equivalent of the £100 I initially borrowed - it buys the same items of shopping - but it "feels" like a lot more.
The compounding of interest of the student loan feels like a lot more too, but it's going up in line with inflation, exactly like the basket of shopping.
I think most people don't find these things intuitive, but it really is just GCSE Maths.
Nevertheless, the truism is, that the less you borrow, the less you will have to pay back and the sooner you will pay it back. Which is why some are suggesting whether it makes sense to max out on the borrowing.