...what would the JSF need to show capability-wise in order to change your mind?My objections are not limited to the plane's capabilities. No capabilities would justify especially egregious costs. A sure-to-be-revised-upward $1.7T is probably egregious enough. Similar criticism applies to the military overall.
...if I was only allowed to have one plane in theater...
This seems an unrealistic limitation. Very few aircraft (or ground craft) can protect themselves from all threats. Perhaps the B2 can, because at night it's apparently invisible? Certainly the Warthog is vulnerable to many fighter jets. (Although in several recent wars, fighter jets have not been a threat?) I assume that even the best 4G fighters are vulnerable to the latest AA weapons. However, I think you have helped me answer your first question. At this time it's clear that no command staff would contemplate operating in a particular "theater" with only F35s as combat aircraft. We would have to consider F35s as capable in some sense, if any forward airbase or carrier group would host them without also hosting F15s, F16s, F18s, F22s, etc. That will never happen, but if it does many people will have to eat crow.
...spend less in the military if it goes to better use instead.
Here is our real disagreement. We should spend less, full stop. Nothing justifies a nation with 5% of the world's population spending 36% of the world's military expenditures. The fact that most of us believe there are such justifications indicates deeper problems. We are constantly gaslit by commercial news media, politicians, academics, movies, etc. that the world is very dangerous for us, that we should fear people who live far way, and that we should probably have special operators and deathdrones terrorizing them on a regular basis just to be safe. Most Americans haven't admitted to themselves that not only do we never enter a war for the reasons cited at the time, not only do we kill and impoverish far more innocent people than ever appear on the news, but having entered those wars we never win, and never accomplish any beneficial military objectives. That isn't to even mention the many wars we fight without admitting them, which of course have had worse results. That isn't to even mention the terrible boomerang effects that all our horrible stupid wars have had on our own society.
Everyone else in the world, who aren't constantly subject to the mixture of news and entertainment that never considers whether we should spend less money killing innocent humans, knows this secret that very few Americans can publicly admit. What drives this flood of misinformation? Adults only get one guess... money! USA cable news industry has almost $6B/year in revenue. USA military spending is over 100 times as much. Few are foolish enough to invoke "Hanlon's Razor" here. We all know that greater spending on political campaign "donations" is firmly in the interest of interested industries. Obviously hiring and editorial decisions in media, as well, can be influenced by advertising and access when resources are so far out of balance. On top of that, the three-week ratings bonanza whenever a new TV war starts are enough to make a network profitable for the whole year.
Start by remembering the unanimous push in the war media to get Saddam's WMDs. Then remember that ObL left Afghanistan entirely less than a month after we invaded. Recall that Qaddafi had outlawed both polygamy and slave markets, which have returned to Libya since his violent overthrow. If our military had fewer resources, it would menace the world less, and regular Americans would be safer.