But whats wrong with PuTTY?, is it missing features?, I mean. like it has full SSH support, its easy to use, and has a non-quirky terminal emulator.. I guess it could have more terminal types, but there are closed source options for that, like Reflections.
Linux in particular is filled with "well enough" solutions that should be striving to be better. Although there's misfires when controversial things like systemd happen, there has been steady improvement in other areas. Imagine if Nginx had never happened, if NCSA httpd was "good enough"?
The Windows user base deserves better than PuTTY.