The argument in favor of the state and local tax deduction is clear and simple: it avoids requiring the taxpayer to make a payment with funds to which he has no legal title. Repealing it would cause even more hardship than, for example, imposing a tax on unrealized capital gains; at least in that case, there is an asset that could be sold to pay the tax. One might as well tax pots of gold at the end of the rainbow.
It appears that the GOP tax reform plan will nonetheless end the deduction, mostly for ideological reasons. The idea behind it is to force blue states to become more like red states: in other words, to change their high taxing and spending model. This is, of course, a violation of the federalist principles that many Republicans pretend to espouse, but only at their convenience; if you don’t believe me, ask any GOP politician whether he thinks abortion should be lawful on demand in blue states if Roe v. Wade is overturned.
There are enough GOP House members from blue states to prevent this from happening. Will they follow the party line at the risk of their own political survival? That is the key question on this front, and I do not pretend to know the answer.