The problem with defining class in terms of wealth is that it basically gives high spenders a cop-out to whine about the fact that they're only middle class because "oh I just have so many expenses"
Guess what? People who only make $20-30k live in the same cities as you do, have kids, have to buy food, have to pay for all of the essentials of life. They are poor. People who make 40-80k or whatever also live in the same cities as you do and have all of the same expenses. They are middle class. You make over $100k, don't care if it's a HCOL area. You elected an expense--living in a high COL area. Whatever your reasoning, including high income jobs available in that area, you're still upper class at an income level that is multiples of the average annual household income. You may elect other expenses, like the fact that your precious little darling couldn't possibly go [to the same inner-city schools that poor and middle class people go to], or you absolutely must have precious little darlings at all (and more than one, even!), or you couldn't possibly live with [the commute that poor and middle class people have in HCOL areas.] Those are all elective expenses. There's nothing wrong with your decision to elect those expenses in lieu of building wealth, but you don't get to cop out of the fact that you're upper class because of it. Embrace it, for f's sake.
When I came out of law school with ~$200k in debt making ~$170k out of the box, I recognized that I was rich, despite the HCOL and the fact that I don't remotely spend like I'm rich. Why? Because I've been able to wipe out debt that it would take an actual poor/middle class person much longer to wipe out. I've been able to save a lot more money that an actual poor/middle class person could save. Sure, I am richer than someone who is just making $100k, and I've also elected to defer certain expenses like buying a house and having kids until I'm on firmer financial footing--but none of that means I'm not rich.
ETA: Getting back to the original thread. Backdoor Roth IRA is an obvious tax avoidance scheme. It shocks me that the IRS didn't clamp down on it as soon as it started--it would be very easy for them to do so. Getting rid of it makes eminent sense. Make an affirmative policy choice to allow contributions, or not, but don't create a tax hole that anyone with a Vanguard account can abuse.