Any such rule is revenue-negative. This would likely mean tax rates would have to rise (unless what we’re really trying to do is provide a tax cut).
The reason is simple:
if people are better off under the current system, they won’t choose to defer income.
If people find out that they’ll pay less tax through deferral, they’ll defer. (one simple way is to defer income from when you have a high tax rate till when your marginal rate is lower-as people in the thread note, when their income is lower).
So overall, you have one group of people who pay as much tax as they do now, and another group that pays less tax.
The OP asks to set aside revenue neutrality, and to look at abuse-- but the two are linked, as if a new rule allows people to dramatically lower their tax burden, then 1) it’s intended as a tax cut, or 2) it allows abuse.
I might note that while the difference may seem unjust to someone who earns nothing in year 1,2,3, and lots of money in year 4, the people who this benefits most (and who will abuse it) are those who earn 1M in years 1,2,3, and 100M in year four-they’re the ones who will benefit most from being able to defer tax, and who will (likely) have some years with large losses from which they can take large deductions if we allow them to shift income to the years in which they have allowable deductions.