I remember reading on Wyden's website that he was proposing to ban all Roth conversions. This is different from what President Obama has proposed in his budget the past few years. That proposal would prevent people from converting after-tax amounts from traditional retirement accounts (ending the backdoor Roth), while allowing pre-tax amounts to still be converted (which would leave the Roth ladder in place).
Obama's plan would be a net positive for the federal budget. The assumption is that people would start contributing to taxable accounts instead of backdoor Roths, which would cause dividend and capital gain tax collections to increase bit by bit every year as that money compounds.
Wyden's proposal would likely look like a net negative under the budget metrics they use. Many (most?) Roth conversions are of pre-tax amounts, which means the government gets their tax money now instead of far in the future when the account owner finally withdraws it. They tend to only look at the impact of a proposal over ten years, so it seems like ending all Roth conversions would be more of an uphill battle than ending only the backdoor Roth conversions.
That said, I have no idea what to expect from the upcoming all-Republican government.
But even if we consider that Wyden's proposal would be adopted in its entirety, it wouldn't be the end of the world for prospective early retirees. I have heard of no serious proposal to prevent early withdrawals entirely, or to increase the 10% early withdrawal tax. So in the absolute worst case, for someone whose entire stash is in traditional retirement accounts, and who plans to spend it all down during the timeframe where the 10% early withdrawal tax would apply, banning Roth conversions would mean your traditional retirement accounts are worth 10% less than they once were. Instead of saving 25x to meet the 4% rule, you'll need to get to 27.5x. That's assuming you'll just make early withdrawals and pay the extra tax. Use SEPP or save some money in Roth or taxable accounts or plan to spend some of that money past age 60, and the damage is less.
Honestly, the loss of Obamacare makes me rethink the feasibility of FIRE much more than any tax plan that I've seen proposed.