I don't know if anyone has mentioned that your money will grow faster in the 401(k) as well. If you are in the 15% tax bracket and put $17,500 into a 401(k), you are saving about $2,500 (15% of $17,500) in taxes. In other words, $2,500 MORE is working for you as an investment. Granted, if you are FIRE'ing only a year or so later it won't have as much time to compound, but if it's growing at 8%, that difference becomes about $2,700 after a year, $2,900 after 2 years, and $5,400 after 10 years.
As you seemed to have guessed, most people here are assuming that they will need a lot less money in retirement than they are making now (myself included), and will only withdraw enough to cover those needs, so their retirement incomes will be lower. Many don't intend to work until they have enough investment income to cover 3-4x their future expenses even though they could be making 3-4x their expenses now. Even if they do intend to make that much, they will not be withdrawing 3-4x their expenses each year.
As for income tax rates being higher, we don't necessarily know that it will happen. It depends on how you intend to FIRE. If you intend to FIRE and live off $20K/year for 2 people, that is considered a low income and I doubt the government would increase income taxes on that level of income. If you intend to retire at the same income as what you make now, for example $70K/year for one person, then you may need to worry about it more. Even in that case, though, by deferring taxes now you are allowing a bigger amount to grow and you can control how much you withdraw (to a certain degree, anyway).