I guess you're worried that Democrats will be in power in 2027 and allow middle class tax cuts to expire?
I find it amazing that you have already, pre-emptively, blamed democrats for potentially, in the future, maybe NOT changing a republican tax bill.
Let's review. The republican tax plan is designed to raise personal income taxes paid by most Americans. In order to trick poor people into voting for higher taxes on themselves, it offers them a temporary tax cut up front which will then phase out into a tax increase over time. This is the Republican plan, written without any Democrats and passed without any Democratic votes, and yet you're blaming Democrats for the way Republicans have written their law.
That's some hardcore tribalism right there.
The scenario you've described would not be "democrats letting middle tax cuts expire" it would be "the republican tax plan going exactly as republicans want it to go." Republicans want to raise taxes on all but the richest Americans, in order to help pay for tax cuts for the richest Americans. It's a fundamentally redistributive tax plan, from the poor to the rich, which also happens to add trillions more of Republican national debt.
Republicans had a genuine opportunity here to reform the tax code. They could have shown some of that fiscal responsibility they're always talking about. They could have actually cut taxes on the middle class. Instead they did neither, and squandered their opportunity.
Maybe in a few decades we'll get Democratic full control of the entire government again, and they can try to pass tax reform that actually accomplishes what everyone says they want, but seemingly refuses to do. God forbid the two parties actually work together on this one, despite their agreement on what the reform should look like.