I think that's a very simpleminded and optimistic analysis, but it's true that abandoning your allies is not a good approach. However, was Ukraine an ally? IIRC the negotiations were still in process. They were in the extremely dangerous position of "holding rich resources and being adjacent to an acquisitive power".
The analogy to Czechoslovakia prior to WWII fails because Czechoslovakia *was* a ally, per the treaty. (See "entangling alliances".)
Ukraine was a friendly nation moving closer to the west (and the US in particular). Not a formal ally, but it had formal interactions with NATO.
I don't think Taiwan is much different. They cooperate, but there's no formal obligation for the US to defend them and the official policy is ambiguity.
And this is one of those cases where I think the "simpleminded" analysis is the right one. Even dictators need to justify their actions to their populace, meaning international politics can be very low bandwidth. The rule "no wars of conquest" was a very simple and effective one. In Russia's eyes this got sullied by the NATO intervention in Yugoslavia. And more likely by the US invasion of Iraq.
If Bush doesn't go into Iraq, and if he played the NATO expansion into Eastern Europe better, I'm not sure Georgia or Ukraine get invaded.
As to Taiwan, again, simplemindedness wins. How far the US goes to defend Taiwan depends heavily on political expectations. And if Ukraine demonstrates that the US has low resolve for defending a friendly nation then China will be encouraged to act, and that expectation of low resolve makes it harder for the US to sustain a defence of Taiwan.
That's why Biden ignored "strategic ambiguity" and said the US would defend Taiwan in 2022. To discourage the Chinese from deciding to invade during the distraction of Ukraine.