As for rebar making concrete structures less durable; yes, that's certainly true for steel rebar. The reason being that it will rust, very slowly at first, but

As for rebar making concrete structures less durable; yes, that's certainly true for steel rebar. The reason being that it will rust, very slowly at first, but once it starts the expansion of the rusted part causes cracks in the concrete which allow more humidity and oxygen to reach the steel, thus rusting faster. This is often called "concrete cancer", and limits the useful lifetime of most modern reinforced concrete structures to between 50 and 250 years (depending on the environment they are in, the forces they are exposed to, and the quality of the concrete they were constructed with). Concrete cancer can be reduced or even eliminated by using rebar material that rusts more slowly (stainless steel) or not at all (carbon fiber), but these are much more expensive of course. There is room for research on other reinforcing materials, but basically nothing with good tensile strength is going to be cheaper than steel and considering the quantities of rebar we use, cost is definitely a major issue. The self-healing nature of Roman concrete might also help here, but the chemistry of concrete and rust formation on embedded steel is complex, and without extensive experimentation right now we don't know if steel embedded in Roman concrete rusts faster or more slowly than in modern concrete (before considering cracks).