One you would remove the value of the degree,
The value of a degree is in what it lets you do not in how much it costs to get it. The skills you acquire are valuable both to the individual and to society.
two it would be discriminatory against those with learning disabilities.
I utterly fail to understand this point. Your statement is as logical as arguing that governments should not fund vaccination programs because some people are allergic to vaccines and can't have them. The reason degrees should be free is the same reason why school education should be free: society as a whole benefits from having highly skilled people available for employment or even who can create new employment.
There are many degrees that have absolutely no value in the modern world.
I would not go that far but I would agree that there are some degrees which are more valuable than others. However this is the great thing about making degrees free: governments can choose how many slots to fund for each program based on the need. The university then selects the best students for those funded slots. Too many english graduates then fund fewer spaces. Not enough teachers then increase the funding for education degrees etc.
This is far better than the current system where many students pick subjects for less sound reasons such as which subject they think is easiest.
Someone earning 21,000 a year only repays 66 a month. There is no reason someone who has taken a serious degree and gotten a job can not pay back that.
You are missing the point. Yes they can afford to pay the amount but the problem is that the loan comes with interest. Someone in a job like teaching gets a reasonable but certainly not high, salary. Given that the interest rates on the load is RPI+3%. On a 36kGBP loan that's just under 2k interest per year which is 166/month. So your person paying 66/month will never, ever pay off their loan. Essentially they will have a permanent extra 9% marginal tax rate throughout their life.
Compare this to those entering a high paid job in finance. They will have a 9% marginal tax rate for a few years until their loan is paid off and then it's over. This means that the loan not only discourages graduates from taking lower paid jobs but the interest means it acts as a regressive tax where those earning less pay more.
In the previous system we had higher rates of tax for high earners and this was used, in part, to fund the university system. Not only was this clearly fairer it did not dissuade people from important jobs with lower pay scales and it reduced the grumbling about tax of those earning a lot because they could see that they had benefited from taxes.