Middle class is being able to live comfortably, get 2 weeks of holiday abroad, afford some but not many luxuries, have a house and enjoy children and hobbies without going bankrupt. Maybe every couple decades you get yourself a nice Audi car.
It is very possible that the middle class is shrinking. But you also have to admit that it's a bit harder to notice it of you're a software engineer in the Silicon Valley on $250k a year and full private life and health insurance.
Even as a software engineer with a decent stint in the top 5% bracket of UK salaries (£70,000) I wonder how the hell am I going to afford a house with more than one bedroom for myself, and I'm already luckier than 95% of the country.