Alliances rely on trust.
NATO allies now know that vast swathes of the American electorate are willing to knowingly vote for probable Russian or Chinese assets, who won't have their back. If not in the next election, possibly in an election with the next few decades.
This has already been factored in. For example, given the timing, I doubt it's entirely a coincidence that the Japanese withdrew from their partnership with Lockheed Martin in favour of cooperation with BAE for a 6th generation fighter. Talk of (and funding for) EU strategic autonomy also increased dramatically after Trump's election. And I know plenty in the US like to complain about the EU not pulling its weight, historically not without reason, but in the longterm the Europeans moving further from the US won't result in more weapons sales for the US. Likely the opposite. Europe first.
When it comes to intelligence sharing, the US electing a Russian asset who shared classified documents with the Russians and likely got foreign assets killed, really isn't something that you can fix in a few years either. The damage is lasting if not permanent. Trump may be gone, but US institutions are still filled with his supporters.
Especially at a time when China is ascendent, it's possible that Trump has permanently fucked the US and that the zenith of US power and influence has already passed. That it is all downhill from here. Behold the decline and fall, let's all hold hands with our backs to the wall.