There would be, if NASA had the budget to plan and fund a couple dozen missions at a time. Going one at a time, with each next mission being uncertain, all they can do is to maximize the amount of science their mission budget buys, and that means building one-off, highly-specialized instruments.

The design they're sticking with is instead Curiosity; the Mars 2020 rover is basically an update of that design with different instruments, better navigation software, upgrades to parts that specifically took a lot of wear on the Curiosity mission, and a cool little helicopter scouting drone.

And rockets too, while firing 30 at the same time for redundancy?

This is not a valid argument at all. You need different designs that will not fall in the same way. We could e.g. send Lockheed, United Space, SpaceX, Russian, ESA and CSA designs for all of the components at the same time, then it is perhaps possible that at least one week make it. The cost will be huge though.

> while firing 30 at the same time for redundancy?

I presume this is a reference to the failed N1 project with it's 30 NK-33 engines? To be fair, the N1 rocket didn't fail because of it's engines, it failed because of numerous organizational failures, including a ruthlessly rushed design process, lack of funding, and most importantly, the death of it's chief engineer, Korolev. The NK-33 rockets were basically the only part of the rocket that actually survived the program-- they're a big part of the current Soyuz stack if I recall.