I wonder how long carbon fiber rebar, or plastic coated steel rebar would last.

Basalt rebar is chemically resistant and doesn't corrode. It does have a lower tensile strength than carbon fiber but is suitable for current designs.

Plastic coated rebar is a product. In theory it can solve the rusting problem, but in practice the coating will get some nicks during installation that can concentrate corrosion leading to point weaknesses. There are additional concerns with the bonding between the concrete and the coated bar.

I don't know how it compares to actual rebar reinforced concrete, but we do use fiberglass reinforced concrete.

Steel and concrete have similar coefficients of expansion. Most other choices don't and so as temperature changes something has to bend or give which reduces life.

Briefly (costs/duration):

normal steel rebar=1

epoxy coated rebar=1.1-1.3 <- terrible in practice

zinc coated rebar=1.2-1.4 <- good

stainless steel rebar=3-4-5 <- very, very good

carbon fiber rebar=? <- AFAIK not used in new buildings, as it has a series of issues in practice, used in some rare cases of consolidation/repair of old buildings, mostly not as rebar but rather as mesh/net.

What is used, not as "main" reinforcement but to have a (much better) mass strength are fibers, they can be either steel or plastic.

I think I do not understand the numbers. You wrote "costs/duration" so higher the number the worse solution (bigger costs or worse durability), yet the descriptions suggests otherwise.

Sorry, numbers are cost only (1 is "normal" rebar steel) order and text are duration ("normal" steel no text, base duration, the terrible in practice means that it doesn't offer longer duration, and may have issues in adherence to concrete).

Not so surprisingly the higher the cost the longer the duration.

There are stainless steel rebar and epoxy coated rebar for structures exposed to the elements. Stainless steel rebar appears to be kind of expensive so its use is limited.

Interesting idea, but carbon fiber would need to be really cheap. Would work well until a critter of some sort evolves that eats plastic

Is there any alternative to steel?

Or, is there any promising technology that allows to use steel bars that have some space inside, so that when they rust, they don't break the concrete?

It's critical to protect the concrete from water.