Well since they haven’t completed the transition in two years like they said they were going to who knows.
Just going by how long the current operating system is supported by older devices is one way to guess. macOS Monterey supports the Mac Pro from 2013. On the assumption that the next version of macOS does not support that 2013 Mac Pro that means 8 years of support. So that means macOS should support the 2019 Mac Pro all the way up to 2027.
The only problem with that is it doesn’t take into affect when hardware is discontinued. I think that would be a more realistic way to calculate how long a device is supported. Basically once these older Intel Macs become ineligible for the newest version of macOS then it will just be Apple Silicon only for macOS because then they don’t need to support Intel. There would be no reason to include support for it if it’s not going to be on an Intel Mac
Just going by how long the current operating system is supported by older devices is one way to guess. macOS Monterey supports the Mac Pro from 2013. On the assumption that the next version of macOS does not support that 2013 Mac Pro that means 8 years of support. So that means macOS should support the 2019 Mac Pro all the way up to 2027.
The only problem with that is it doesn’t take into affect when hardware is discontinued. I think that would be a more realistic way to calculate how long a device is supported. Basically once these older Intel Macs become ineligible for the newest version of macOS then it will just be Apple Silicon only for macOS because then they don’t need to support Intel. There would be no reason to include support for it if it’s not going to be on an Intel Mac