There are plenty of split mechanical keyboards similarly adjustable as the M15 for a couple hundred dollars. Mechanical switches aren't the same as IBM's buckling spring.
Maybe Unicomp will one day be able to afford new tooling.
The buckling springs do make a nice sound, but the 80 gram activation pressure is painful when Topre keyboards are 35/45 and Cherry Blues are 50-ish.
I was a big Model M fan for a long time, but Topre keyboards are a lot more comfortable. And they still sound pretty excellent.
(I also like Cherry Browns, because the feeling before bottoming out is more noticeable than on the Topres. But the Topres have better build quality and ... I just like them better :)
Maybe Unicomp will one day be able to afford new tooling.