Have to take issue with this:
"So why are they not in the higher paid jobs already, if they have the qualifications, skills and willingness?"
I have a 1st class honours degree (graduated 2nd in my year) and masters and habitually worked 60-hour-weeks at one point. I think my intelligence and diligence is demonstrated. I am still poor. Caring for an elderly relative and then becoming a single parent has severely curtailed my earning potential. While things will improve as my children get older, the damage done is not totally redeemable and I will never become a higher rate tax payer. I'm not after sympathy, BTW, as I am generally happy and have learned to count my blessings.
My rambling point is that earnings are not a reliable indicator of a person's worth. Among higher-rate payers will be those who have worked damn hard to get those and those who have just been lucky. Similarly, among the poor will be those who have been dreadfully unlucky and those who are just lazy. It works both ways.
It's worth remembering though that an extra percentage point of tax on a higher rate earner, while it may be extremely problematic for them as an individual, is never going to cause the same degree of difficulty as a cut in benefits for someone at the bottom. There are always more options available to you if you are better off. I work full time. My friend is on benefits. After I've paid childcare and mortgage, we are left with the same amount to spend on bills, food, etc. However, my money goes further because I have access to reasonable credit and can do simple things like pay gas/electric by direct debit (whereas she has to use pre-payment card etc, which has a much more expensive tariff). My washing machine breaks down? I can get one on credit for 0% interest for 6 months, giving me 6 months to save up. Hers? She has to take out a Provident loan at 100%+ APR (I kid you not) and starts paying interest immediately. Over time, these things make a BIG difference.