As I'm doing the math of when we can expect to actually see a Mars mission, I'm finding it difficult to believe we can get there until we complete several additional programs to prove technologies we'll need to get there.
I'm talking programs on the order of the ISS, Lunar Gateway, or a Moon base, each of which would take a decade, realistically.
The problem is that there are so many things we'll have to do on a Mars mission that we've never done before. And if we just try these things for first time on a real mission, and they fail, there's no getting home for years. Once we launch, they have to work.
Am I missing something here, or are we going to need multiple decade-long programs to try out these technologies and get them to work reliably?
Spacecraft robustness and independence
The spacecraft that gets us to and from Mars will have to function independent of Earth for multiple years, continuously. Having operated the ISS for almost 2 decades, this is the technology we have the most experience with. But surprisingly, we still haven't demonstrated it to the level required for a Mars mission.
Apparently there are components on the ISS with a mean time between failure of less than 6 months. The life support systems on the ISS are also far from a closed system, only recycling 50% of its oxygen and 90% of its water. 
Building a base on a planetary surface
Landing a multi-ton habitat on a body that poses such an EDL challenge as Mars is something we've never come close to.
Also, the problems with lunar dust in Apollo were notorious. Yes, we've learned our lesson and they've come up with solutions like those spacesuits that attach to the habitat and never come inside. But again, we've never actually tried them out in a realistic scenario.
And that's only scratching the surface of the unknowns. Living on a planetary body long-term is probably the biggest first in the whole mission.
In-situ resource utilization
We've drilled into some rocks, but we've never gotten close to the sort of mining and material handling required to extract water from rocky ice deposits. And mind that most plans require this to be done robotically, performed by a return craft that we send out ahead of time.
Or are we just going to end up bringing the hydrogen ourselves and only use the CO2 in the atmosphere to get methane fuel? How much mass does this compromise cost us?
Of course, there are many more issues on this scale. For instance, we still don't even know how much 1/3rd gravity mitigates the health effects of weightlessness. And no one's ever lived in space for half as long as the Mars astronauts will have to.
What's the plan
I've read plenty about the unknowns but it's unclear that there's a solid plan for addressing all of them without methodically proving each technology in space or on the Moon. Is that really the only way this will get done, or am I being overly pessimistic?
Are there maybe examples from the Apollo era of how this can be done on the go? And in ways that aren't invalid in a multi-year Mars mission?
Edit: Here's the source of that first image.