What ever happened to Tandem and NonStop OS 2125
The device handlers would need rewriting to allow for the differences in hardware interfaces. The loader (and compliers) would need rewriting to allow for the different instruction set. The scheduler would need modifying due to the different change process procedure. Nothing difficult there.
What ever happened to Tandem and NonStop OS 2126
Three things here. Firstly, your customers are going to be seriously peeed off if the system is underpowered. But that is not as imporant...
As for the applications, we are talking porting the DEC ones V. writing the Windows ones from scratch. Advantage DEC.
The Rainbow was built to very much higher standards than other PCs. My employer had to test some for a military project. The Rainbow was the only one to pbutt full environmental category 2 tests. Unlike Army vehicles most companies heat their buildings and have good power supplies, permitting the use of cheaper components.
The Rainbow should have replaced the Alpha. RISC is an engineering compromise rather than an aim in its own right. RISC needed fewer chips than CISC. When CISC could fit on a single hardware module RISC was marginal. With the introduction of a single chip 32-bit CISC CPU (the Motorola MC68020) in 1984 RISC was obsolete.
In simple terms the difference between manufacturing a single chip VAX with 64 bit addressing and the Alpha is $1 per chip.
Rainbows were computers in their own right. Running terminal emulators was needed but being members of the VAX cluster in their own right would have been nice. Rainbows were competing with the PDP-11, not its terminals.