The switch from from DOS-based Windows to NT took place over many years: development started in 1989, the first release was in 1993 and the switch only really completed in 2001 when Windows XP was released, which was the first NT release targetted at consumers. That's well over a decade. Microsoft bet a huge amount of resources on NT because they could see that DOS was never going to work on in the long term, especially on servers. There's no reason for them to spend so much resources and time on a switch a *nix kernel when there's (currently) nothing like that at stake with the NT kernel.
Longhorn was the codename for Vista, which just used the same NT kernel as before (with some feature improvements, just like in every new Windows release).
Actually there were a couple of other reboots, but staying on the same NT kernel model, one being the kernel refactoring around Windows 7 release, and during Windows 10 it got the capabilities of split personality with secure kernel, driver guard and ability to run containers directly on top of Hyper-V.
Longhorn was the codename for Vista, which just used the same NT kernel as before (with some feature improvements, just like in every new Windows release).