It is true that some soft processors have become obsolete (Nios is now Nois II). So you have reason to doubt that another one would be successful.
However, both MicroBlaze and PicoBlaze are doing very well, azs measured by the thousands of downloads. Each download may be just one person "kicking the tires" or it may be one download that gets placed in tens of thousands of boards, we have no way to know.
We have decided that whatever we support, Power PC, MicroBlaze, or PicoBlaze, it can not change such that old code doesn't work! That is the death of any processor. Rather, all old machines may evolve, but they must evolve in a way that allows old code to move with the changes (similar to Intel and the x86).
A processor needs to exist, it needs an eco-system (compilers, linkers, loaders, debuggers, etc), it needs to meet the needs (speed, power, cost), and it needs to last the intended length of the company that wishes to use it (it is a printer every 6 months? or a new network switch every three years? or a new spacecraft once a decade?).
Many questions. Look up the answers,