Politically, a Republican president with a sub-40% approval rating is the best thing that could happen to Democrats.
In terms of policy, it's a tougher question. Trump's win was an environmental catastrophe, the judicial appointments will make conservative rulings for years, and the repeal of the individual mandate could be tough to reverse and cause major damage to insurance markets.
However, most of the provisions of the recent tax bill should be easy enough to reverse in 2021 or 2025, and the new funding allowed by the reversal of those tax policies could pay for major legislation such as an expansion of Medicare/Medicaid, a green stimulus," or universal pre-k. Also, the Supreme Court is still in play in the near/medium term until Kennedy retires.
This makes single-payer inevitable, especially if we get a Dem president in 2020 with large majorities in both houses (which is looking more and more like Trump's legacy).
And also, the Supreme Court is always going to be in flux to some degree: yes, Kennedy and Ginsburg are both in their 80s, with Breyer not far behind, but Alito (at 67) and Thomas (at 69) but are no spring chickens either.
