There are certainly poor teachers out there, but it is difficult for parents to make a definitive judgement because they just don't have enough info or the full picture to be able to say so accurately.
I think if parents have concerns, they should put it in writing to the school and ask what the school procedure is for such complaints. You then do have to trust the school (who have or can access the full picture) to follow up and choose appropriate action. As a parent you won't be told if and what any action is. You can of course complain again, but in the end, it is rightly not the job of parents to decide staff should go, only to report their personal experiences.
Re the question of whether staff are got rid of.....well yes they are, through various means. As others have said, it can take a while to establish if someone is officially incompetent - most people who face complaints might have some issues but these don't make them incompetent or bad teachers overall. If there's an individual issue it can usually be easily addressed. Broader failures, such as subject knowledge, class control, not covering the course or preparing exam classes etc are often hard to pin down and take a long time. Schools do get a sense though where these are serious and targets can be put in place as part of a programme which is effectively disciplinary. People need a chance to improve and help to do so.
What people often report happens, is if a school decides a staff member isn't up to the job (often justified but sometimes unfair) then what they often do is set a targets and monitoring regime which is unpleasant, unachievable and involves constant checking so a horrible working experience is created for that memeber of staff - with the goal being that they won't have to sack them (which will take ages and ages) but the staff memeber will be effectively driven out and resign. There are few actual sackings for incompetence but people are driven out - sometimes with good references to just get rid of them, sometimes after being off long term with stress and sometimes after a long improvement process which is unpleasant.
In my view, schools are not always good at addressing problems early on. When they do, in my experience, most are minor and sorted quickly, but some which are serious are not handled well, because the school essentially want to get rid of someone and don't always treat that person right in the process. It is absolutely right for children's education that incompetent people should be removed, but processes and protocols should be used correctly.
Rather long winded, but yes, every year a number of people are forced to leave...and it will be far more than are actually sacked. Many will go and work in another school.