ID.me is already plugged in to IRS' platform, and is used today for all IRS account services to prevent fraud (return fraud being previously very prevalent [1], but there are other threat actor concerns with regards to IRS and identity). Throwing more people at Login.gov will not cause a solution to be arrived at tomorrow [2] [3]. It's suboptimal, but the world runs on duct tape, and a fix is underway. There is always risk, it is about managing that risk effectively.
Personally, I believe saving tens of billions of dollars in potential fraud costs every tax filing season buys a lot of tolerance to people complaining about the interim solution. The complaints are valid, but so are the attempts at fraud. And so, we balance the risk of the implementation.
[1] https://www.justice.gov/tax/stolen-identity-refund-fraud ("The IRS estimated that during the 2013 filing season alone, over 5 million tax returns were filed using stolen identities, claiming approximately $30 billion in refunds. The IRS was able to stop or recover over $24 billion of that total, or approximately 81% of the fraudulent claims.")
[2] https://en.wikipedia.org/wiki/The_Mythical_Man-Month
[3] https://en.wikipedia.org/wiki/Brooks%27s_law
(interviewed with a team that worked on fraud prevention at IRS as part of a US Digital Service interview)
