As others have said there's a difference between class and a comfortable lifestyle. I know several people who have a very comfortable lifestyle with expensive cars, holidays, theme park trips for their children and large houses but no way are they middle class. Nothing wrong with that, well done them that they have worked hard, often in their own trade type businesses, and can give their children maybe what they didn't receive in their own childhood.
I would argue that actually if you do have your own business, by using a good accountant, you can certainly achieve a more affluent lifestyle than someone on PAYE. You can claim all sorts of allowances which significantly reduce your tax bill. Also by paying yourself a small salary and supplementing your income by paying yourself dividends this avoids paying high National Insurance bills too.
Both my husband and I had fathers who were company directors for their own businesses. My husband and his brother both went to very good public schools - I passed the 11+ and went to a good all girls grammar school. We both had middle class childhoods - e.g. having telephones and cars before the majority of people in those days. We had holidays abroad, comfortable houses and were encouraged to appreciate and engage in cultural activities and the arts.
However both father-in-law and my father were from poor backgrounds with a very large number of siblings - through hard work and enterprise they got themselves away from their challenging beginnings. They wanted better for their children and were determined to give us both the stepping stones to achieve it.
Because we grew up in the times before everything became so expensive, we pretty much achieved our comfortable lifestyle on pretty average earning jobs - my husband in public sector management and me in various jobs - banking and latterly in education. We have decent pensions and so are still comfortable in retirement.
We try and help our only child and her little family where we can. Her husband has a very good job in banking middle management and previously my daughter was also middle management in a very stressful job for an American insurance company. No way are they as comfortable as we were, because exactly of the circumstances the Opening Poster describes. Since having the children my daughter has left her stressful job and gone part time, doing unsocial hours so work doesn't impact her time with the children. This has been invaluable during the holidays and we've all had a wonderful time together. But has she changed class now as her job is in a supermarket?!!!!! Are her family still middle class because of her husband's job and earnings or not because of her job?
So we gladly pay for days out, pocket money for the children, and other treats. I also passed on some inheritance from my father to enable them to put down a decent deposit on a house. We also pay for extension activities for the grandchildren, e.g. science club and residential trips.
Their house is a 3 bed semi, not the large detached we have. I think they have a fairly comfortable lifestyle with a bit of supplementation from us - but are they middle class? If you determine being middle-class by the size of their house, then obviously not. Maybe class is an outdated construct (apart from the aristocracy and Royals) and it's not helpful to categorise people by class any more. But of course the point of this thread about the difficulty of achieving a comfortable lifestyle absolutely holds good.
However as Oscar Wilde would say, knowing 'the price of everything and the value of nothing' there's more to a comfortable lifestyle than just income alone -so many things - like modern medicine, IT, housing standards for the majority and benefits are so much better now than in the past.