I think that the difference in the amount you can spend without it seeming like a huge gaping lifestyle gap is enormous, right?
Growing up, I think you'd have said my family was middle class. We didn't have much money at all, but you wouldn't see that from the outside? Dad would paint the walls, and mend the cars. Holidays involved major major bargain hunting, so we might find a cottage in France plus a ferry for say £700. We wouldn't eat out at all, so the rest of the costs were just a bit more than living at home. Lots of our stuff was nice quality, but bought second hand. Cars were bought for maybe £3k, but carefully vetted by dad and then driven for another 80k miles, with them only visiting garages for a big repair or an MOT.
Compare that to running a new lease car at several hundred pounds a month, plus if you pay people to do your DIY that's hundreds of pounds a year. Decorating may only need doing once a decade, but that's thousands for a whole house's worth. Holidays could be thousands different each time. But both families would have "been abroad". If you then apply that to every single thing - being really really careful Vs thinking "oh I won't worry" you can soon eat up a vast difference in income.
Add in two in private school (£2k a month minimum?) And that's then another £40k of gross salary eaten up.
I think it's really easy to spend to your means, especially when often jobs that pay well eat a lot of brain power so you don't have it to spend on canny-ness. I used to help search out holidays and because I was just at school and had holidays etc I didn't mind wiling away evening after evening googling and researching and finding the best, cheapest deals. Now I work full time, I don't want to spend precious annual leave doing that, and it's appallingly easy to go "eugh, I can't spend the time on that' and just throw money at the problem instead of using time to get the same result for less money.
Sorry, I've got somewhat off topic.
I've also not given a number for a "decent" salary, partly because it varies so much depending on where you live.
In my current town, you can rent a 2 bed for £650 a month. Where I lived during my uni years it would be £1300. That means if you're one person you can afford a house with a spare room, holidays and a car on maybe a £25k salary. You feel pretty comfortable. At £1300 you have to be earning circa £40k to be able to do that. That's a huge variance.